Furnace error code chart

Per the Goodman service manual, an internal control failure or a blown board fuse will cause the diagnostic LED to provide no signal. It can also simply mean the board isn't getting power.

On Goodman's single-LED control boards, a diagnostic light that stays on without flashing means normal operation. The service manuals list no corrective action for it, so it isn't a fault code. A GMEC96 is different: its service manual lists a solid red LED as a control board internal fault.

The control tried to light the burners three times without proving flame within 4 seconds of the gas valve opening, then locked out. The code is one short flash followed by a long pause. It auto-resets after 1 hour.

The control sees the pressure switch closed while the induced-draft blower is OFF — it waits until the fault clears. Officially: a faulty pressure switch or shorted wiring.

After the induced-draft blower starts, the pressure switch never closes. The control locks out, then auto-resets and retries. Code: three short flashes and a pause.

The high-limit opened — the furnace overheated. The circulator blower runs until the limit closes. Code: four short flashes.

The control detects flame while the gas valve is de-energized. Blowers run continuously as a protection. Official causes: a short to ground in the flame-sense circuit, miswiring, a lingering burner flame, or a slow-closing gas valve.

The flame-rollout control opened — flames left the burner area at some point. The circulator blower is energized full-time. Rollout switches usually need a manual reset, but the cause must be found first.

The furnace keeps running, but the flame sensor's microamp signal has dropped below spec — a warning before ignition lockouts start. Official: coated/oxidized sensor, a badly positioned sensor, or a lazy flame from poor gas pressure.

Official wording: improperly connected igniter, bad igniter, or poor ground to the equipment.

Official: the 115-volt line polarity is reversed or the unit ground is poor. The furnace won't heat until wiring is corrected.

Three failed ignition cycles — the control can't establish measurable combustion and locks out. Same reset as flash models: thermostat OFF 5–20 s, or kill power ≥5 s.

The low-stage pressure switch reads closed while the inducer is off. The control waits until the fault clears. Cause per manual: faulty switch or shorted wiring.

The low-stage pressure switch doesn't close within 5 minutes of the inducer starting → 1-hour lockout, then auto-retry. Official causes: disconnected hose, faulty switch or wiring, blocked venting.

One of three protective limits opened: primary (overheat), auxiliary, or flame rollout. Blowers run until it closes. Official causes: dirty filter or duct restriction, faulty limit, blower issue, blocked flue.

Flame detected while the gas valve is de-energized. Blowers run continuously as protection. Same emergency profile as 5 flashes on flash-code models.

The control detects its onboard fuse open and inhibits all operation except displaying the code. Official probable cause: a short in the low-voltage wiring.

Furnace runs, but the flame-sensor microamp signal fell below spec. Official: coated/oxidized sensor, wrong sensor position, or lazy flame from improper gas pressure.

Official: check for a broken or improperly connected igniter; check for proper ground and correct.

The high-stage pressure switch is stuck closed when high stage is commanded — the furnace runs on LOW stage only regardless of thermostat demand.

High-stage switch won't close. Official checklist: blocked or pinched switch hose, blocked flue and/or intake pipe, blocked drain, weak induced-draft blower, or a failing switch.

115V or 24V power leads are reversed — the furnace refuses to run. Same fix family as the continuous-flash code on flash models.

The control has no shared data set (it drives the variable-speed motor and airflow math). The furnace cannot function until a model-specific memory card loads the data. Display shows ON when accepted.

The control rejected the shared data set — memory cards are model-specific and a card for a different model will be refused.

The control can't detect the ECM blower running when it should be. Official causes: loose/disconnected wiring between motor and control, an open inductor (¾ HP and 1 HP motors), or a failed ECM motor.

The furnace control can't talk to the ECM blower — the furnace won't operate. Causes: loose harness, failed motor, or failed control.

The connected motor's horsepower doesn't match the shared data set — the furnace refuses to run. Fix: correct motor, or correct data via the proper memory card.

The ECM hit a power, speed, or temperature limit and reduced output — the furnace keeps running at reduced airflow. Officially caused by blocked/clogged filters, restrictive or undersized ductwork, blocked inlet, or high ambient heat.

The ECM motor sensed a loss of control or became overloaded and shut down.

The ECM blower motor failed to start 10 times in a row, and the furnace won't operate while the motor can't start. The service manual's probable causes are an obstruction in the blower housing or wheel, seized motor bearings, or a failed blower motor.

The ECM blower motor shut itself down because line voltage was too high (over 140 VAC) or too low (under 70 VAC), or because its power module overheated. The motor restarts once the condition clears, but the furnace can't run while the motor is off. For overheating, the manual names high ambient temperature or heavy motor loading as the likely cause.

The ECM blower motor didn't get all the information it needs to operate, an event kept it from running, or it failed to start 40 times in a row. Probable causes in the manual: an intermittent wiring connection between the control and the motor, a furnace control error, or any of the conditions behind codes b4, b5 and b6.

Indoor airflow fell below the minimum the control calculates from its shared data. The furnace keeps running with the reduced airflow and, if it was on high stage, steps down to low stage to try to recover. It stops if airflow drops to 0 CFM. The manual's probable causes are blocked or clogged filters, blocked or restrictive ductwork, and undersized ductwork.

Normal state. It also confirms the control accepted memory-card data after a board or card procedure.

IdL is the normal idle display on supported Goodman furnaces. The control is powered, waiting for a thermostat call, and ready to receive thermostat inputs. It is not a fault code.

The GMEC96 control is detecting flame current when no flame should be present. This code belongs to the GMEC96 tri-color control; it is not the same as the one-flash ignition-lockout code used by older Goodman boards.

The official GMEC96 wiring legend identifies two red flashes as a pressure switch stuck closed or an inducer error. The switch is reporting draft before the control expects it.

On the GMEC96 control, three red flashes mean the first-stage pressure switch is stuck closed or the inducer circuit is reporting the wrong state. Match the model and the wiring diagram on the blower door before using this definition.

The limit circuit opened because the furnace became too hot or a limit failed. The GMEC96 service manual says repeated limit openings can put the control into a one-hour lockout.

The control detected an open flame-rollout circuit or open fuse. The service manual also explains that an open limit lasting 15 minutes is stored as this five-flash fault.

The pressure-switch signal dropped out often enough during a heat cycle that the GMEC96 control locked the furnace out. This points to an intermittent draft problem rather than a steady switch state.

The furnace exhausted its ignition retries and entered external lockout. The burners did not establish a proven flame within the allowed attempts.

The burners lit, but the control lost flame proof repeatedly and used up its recycle limit. That is different from seven flashes, where ignition never completed successfully.

The GMEC96 wiring legend assigns nine red flashes to improper grounding or reversed 115-volt polarity. The control will not operate correctly until the line wiring is corrected.

The GMEC96 control is detecting gas flow when the thermostat is not calling for heat. The official wiring legend treats this as a separate ten-flash fault, not a normal operating light.

The limit switch circuit has stayed open for more than 5 minutes. The GMEC96 troubleshooting chart lists the symptom as the furnace not operating while the inducer keeps running, and its check step is to reset main power and confirm the indoor blower operates.

The control board's igniter relay is not energizing the hot-surface igniter. The GMEC96 troubleshooting chart gives a single check for this code: replace the control board.

On a GMEC96, a red LED that stays on while the furnace won't run means the control board has found an internal fault. Goodman's troubleshooting chart lists one fix: replace the control board. The code list labels it an internal, gas valve (GV), microprocessor and frequency check error. The control also lights the red LED at power-up while it runs its self-check, before it goes to standby. On older single-LED Goodman boards, a steady light means normal operation, so check your model before relying on this meaning.

A rapid red flash means the GMEC96 control has found a twinning error, and the furnace won't operate. Twinning wires two furnaces of the same model and size together through their TWIN terminals so their blowers run in sync on one duct system. Goodman's chart gives one check for this code: correct the twinning wiring.

Three double red flashes mean the GMEC96's second-stage (high-fire) pressure switch is stuck open or there's an inducer error, and the furnace won't operate. Goodman's chart says to verify the switch and its tubing, and to confirm the inducer is running and pulling enough vacuum to engage the switch. Count the pairs: three double flashes are a different code from three single red flashes (first-stage switch) or six red flashes (pressure switch cycle lockout).

Three amber flashes mean the GMEC96 control is getting a second-stage heat call (W2) without a first-stage call (W1). The control ignores that call, so the furnace won't heat from it. Goodman's chart points to a faulty thermostat, faulty thermostat wiring, or improperly connected thermostat wires. One or two amber flashes are different: they show a normal first- or second-stage heat call.

Four amber flashes mean the GMEC96 control is getting a cooling call (Y or Ylo) without a fan call (G), and the furnace won't operate. Goodman's chart points to a faulty thermostat, faulty thermostat wiring, or improperly connected thermostat wires. In normal cooling the thermostat sends G along with Y or Ylo, and the control shows one or two green flashes.

Rapid amber flashing means the GMEC96 control is reading a low flame sense current. Goodman's troubleshooting chart lists it under a furnace that won't operate and says to check for a dirty or coated flame rod, a poor flame, improper gas pressure, or a poor flame sense wire or connection. The service manual adds that contaminated fuel or combustion air can leave a nearly invisible coating on the flame sensor that weakens the signal.

The modulating control exceeded its allowed ignition retries or flame recycles on a single call for heat and entered Soft Lockout — E0 flashes during the lockout period, then the control retries.

The low-stage pressure switch reads closed at the start of the heating cycle, before the inducer runs.

The low-stage pressure switch (PS1) failed to close within 30 seconds of the inducer starting — the manual's checklist starts with a blocked pressure-switch hose.

The primary high limit opened — insufficient conditioned air is moving over the heat exchanger. Note this differs from GMVC96-generation boards, where E3 covers three limit types.

Flame sensed while the gas valve should be closed — official causes start with a short to ground in the flame-sense circuit or a slow-closing gas valve.

The board's low-voltage fuse is open — almost always a short in the 24V (thermostat) wiring.

Flame-sense microamps dropped below spec — the control has a built-in warning before lockouts begin. Official first cause: coated/oxidized flame sensor.

The control flags the igniter circuit or the equipment ground.

The high-stage switch reads closed at the start of the heating cycle.

PS2 won't close — blocked hose, flue, or drain per the manual. Note: E9 also displays while the control runs its calibration routine; if calibration completes, it clears on its own.

Line-voltage hot/neutral is reversed (or ground is poor) — the furnace refuses to run until corrected.

The inducer drew excessive current or lingered on when it shouldn't — a failing induced-draft motor signature.

The manual-reset flame rollout switch is open, and the furnace won't run. Possible causes listed in the service manual: an orifice plate out of position, a blocked heat exchanger, misaligned burners, or a defective heat exchanger.

The control detects wiring to the furnace that doesn't match the expected configuration — commonly after thermostat or accessory work.

The auxiliary input circuit is open — an external safety device wired into AUX (condensate overflow switch, etc.) has tripped.

The control can't detect the ECM blower running when commanded — loose wiring at the motor or a failed motor.

The control lost communication with the ECM blower — the furnace won't run without it.

The installed motor's horsepower doesn't match the shared data set — usually after a wrong replacement motor or wrong memory card.

The ECM hit an operating limit and reduced output — the furnace keeps running with less airflow. Blocked filters are the manual's first cause.

The blower tripped on overcurrent or lost rotor sync — abnormal motor loading or a sudden change in load.

The blower can't turn — obstruction in the blower housing or a seized motor.

Over/under line voltage to the furnace, or the motor exceeded its temperature limit.

The control failed to deliver a complete parameter set to the ECM — a control-side error.

Delivered airflow is lower than the demand target — blocked filters are the manual's first cause.

The control has no shared data set (NO NET) — the furnace can't run until a model-specific memory card loads it. Display shows ON once data is accepted.

A configuration status code — conventional 1-stage compressor cooling with dehumidification is active. Informational, not a fault.

Configuration status — conventional 2-stage compressor cooling with dehumidification. Informational, not a fault.

The control rejected the memory card — cards are model-specific and a mismatched card will be refused.

Three failed ignition cycles — the control can't establish measurable combustion and locks out. Same reset as flash models: thermostat OFF 5–20 s, or kill power ≥5 s.

The low-stage pressure switch reads closed while the inducer is off. The control waits until the fault clears. Cause per manual: faulty switch or shorted wiring.

The low-stage pressure switch doesn't close within 5 minutes of the inducer starting → 1-hour lockout, then auto-retry. Official causes: disconnected hose, faulty switch or wiring, blocked venting.

One of three protective limits opened: primary (overheat), auxiliary, or flame rollout. Blowers run until it closes. Official causes: dirty filter or duct restriction, faulty limit, blower issue, blocked flue.

Flame detected while the gas valve is de-energized. Blowers run continuously as protection. Same emergency profile as 5 flashes on flash-code models.

The control detects its onboard fuse open and inhibits all operation except displaying the code. Official probable cause: a short in the low-voltage wiring.

Furnace runs, but the flame-sensor microamp signal fell below spec. Official: coated/oxidized sensor, wrong sensor position, or lazy flame from improper gas pressure.

Official: check for a broken or improperly connected igniter; check for proper ground and correct.

The high-stage pressure switch is stuck closed when high stage is commanded — the furnace runs on LOW stage only regardless of thermostat demand.

High-stage switch won't close. Official checklist: blocked or pinched switch hose, blocked flue and/or intake pipe, blocked drain, weak induced-draft blower, or a failing switch.

115V or 24V power leads are reversed — the furnace refuses to run. Same fix family as the continuous-flash code on flash models.

The control has no shared data set (it drives the variable-speed motor and airflow math). The furnace cannot function until a model-specific memory card loads the data. Display shows ON when accepted.

The control rejected the shared data set — memory cards are model-specific and a card for a different model will be refused.

The control can't detect the ECM blower running when it should be. Official causes: loose/disconnected wiring between motor and control, an open inductor (¾ HP and 1 HP motors), or a failed ECM motor.

The furnace control can't talk to the ECM blower — the furnace won't operate. Causes: loose harness, failed motor, or failed control.

The connected motor's horsepower doesn't match the shared data set — the furnace refuses to run. Fix: correct motor, or correct data via the proper memory card.

The ECM hit a power, speed, or temperature limit and reduced output — the furnace keeps running at reduced airflow. Officially caused by blocked/clogged filters, restrictive or undersized ductwork, blocked inlet, or high ambient heat.

The ECM motor sensed a loss of control or became overloaded and shut down.

The ECM blower motor failed to start 10 times in a row, and the furnace won't operate while the motor can't start. The service manual's probable causes are an obstruction in the blower housing or wheel, seized motor bearings, or a failed blower motor.

The ECM blower motor shut itself down because line voltage was too high (over 140 VAC) or too low (under 70 VAC), or because its power module overheated. The motor restarts once the condition clears, but the furnace can't run while the motor is off. For overheating, the manual names high ambient temperature or heavy motor loading as the likely cause.

The ECM blower motor didn't get all the information it needs to operate, an event kept it from running, or it failed to start 40 times in a row. Probable causes in the manual: an intermittent wiring connection between the control and the motor, a furnace control error, or any of the conditions behind codes b4, b5 and b6.

Indoor airflow fell below the minimum the control calculates from its shared data. The furnace keeps running with the reduced airflow and, if it was on high stage, steps down to low stage to try to recover. It stops if airflow drops to 0 CFM. The manual's probable causes are blocked or clogged filters, blocked or restrictive ductwork, and undersized ductwork.

Normal state. It also confirms the control accepted memory-card data after a board or card procedure.

Official: check for 115VAC at L1/L2 and 24VAC at SEC-1/SEC-2. No light means the board isn't powered.

Official: line-voltage polarity is reversed — and the control blocks heating until it's fixed. (If the furnace is twinned, see the twinning-kit instructions.)

1 short + 1 long flash on recall = nothing stored. Codes self-erase after 72 hours (and on RED-LED boards, whenever power is interrupted).

The furnace was powered up in the middle of a call for heat (R-W closed), so the blower runs 90 seconds first. Informational, not a fault.

A limit, draft safeguard, flame rollout, or blocked-vent switch stayed open longer than 3 minutes → lockout. Auto-resets after 3 hours. Root causes = see code 33.

The system failed to ignite and prove flame in 4 attempts. Auto-resets after 3 hours. Root causes = see code 34.

Official: mis-wired gas valve, a stuck-closed gas-valve relay on the control, or a shorted gas-valve wire. This one does NOT auto-reset.

Flame is proved while the gas valve is de-energized; the inducer runs until the fault clears. Official causes: a leaky or stuck-open gas valve.

Official: obstructed pressure tubing or a pressure switch stuck closed.

A short in the 24VAC wiring blew the board's automotive-type 3-amp fuse. Official: replace with a 3-amp fuse ONLY.

If the switch stays open over 5 minutes, the inducer rests 15 minutes and retries. Official checklist: excessive wind, low inlet gas pressure (if LGPS used), disconnected or obstructed pressure tubing, defective switch.

A limit, draft safeguard, flame rollout, or blocked-vent switch is open. The blower runs 4 minutes or until it closes; open >3 minutes escalates to lockout code 13. Rollout and BVSS switches need a manual reset.

No flame proved during the trial for ignition; the control retries 3 more times before lockout 14. Official checklist: flame sensor must not be grounded, oxide buildup (clean with fine steel wool), proper microamps (0.5 min, 4.0–6.0 nominal), gas valve on, igniter health, inlet gas pressure.

Auto-resets after 1 hour. Official causes: gas-valve relay stuck open, flame-sense circuit failure, or a software check error. Reset power to clear; replace the control if it repeats.

On the two-stage variable-speed control (58MVP, 58MVB, 58UVB), the blower failed to reach 250 RPM, or failed to talk to the control within 30 seconds of being switched on, in two heating cycles in a row. Official: the lockout auto-resets after 3 hours, and the root causes are the same as code 41.

Official: if code 25 flashes only 4 times at power-up, the model plug PL4 is missing and the control is running on the model stored in memory, and the furnace operates properly. If it flashes continuously: PL4 missing with no valid model in memory (typical after a replacement board), a thermostat call while setup switch SW1-1 or SW1-6 is ON, both switches ON together, or two different furnace models twinned.

On this two-stage variable-speed control, code 31 means the high-heat pressure switch didn't close on a call for high heat, or opened during high heat. Carrier's guide adds that the control relay may be defective or the gas valve miswired, and points to code 32. After a shutdown, the control either retries in 2 minutes or limits the next cycle to high heat, depending on when the fault happened. If the switch closes during low heat instead, the control flashes 31 and moves up to high heat. Single-stage 58STA and 58STX furnaces use code 31 for a different fault.

On the two-stage variable-speed control, the low-heat pressure switch did not close within 45 seconds of a call for low heat, or it opened during low heat (a low gas pressure switch opening reads the same). The control shuts down, waits 2 minutes and retries. If the switch opens within 5 minutes after ignition, the next cycle runs in high heat only. Official checklist: vent sizing, plugged condensate drain, low inlet gas pressure (if LGPS used), excessive wind, restricted combustion air, pressure switch wiring, failed or out-of-calibration switches, water or sag in the vent piping, disconnected or obstructed pressure tubing.

On the two-stage variable-speed control, the blower failed to reach 250 RPM or failed to talk to the control within 30 seconds of being switched on (10 seconds during steady-state running). Official first checks, power off: a blower wheel rubbing the housing, a loose wheel on the motor shaft, and the wiring from the control to the motor. After that the flowchart is a voltmeter test at each plug that ends at the blower control module, the motor, or the furnace control.

On the two-stage variable-speed control, the inducer did not start within 20 seconds of a call for heat, its RPM is outside the valid range, or the RPM signal was lost for 5 seconds while running. Official checklist: proper vent sizing, restricted combustion air supply, failed inducer motor, improper motor wiring.

On the two-stage variable-speed control, a call for high heat closed the high-heat pressure switch but the low-heat switch stayed open. The control shuts down, stops the inducer, keeps the gas off, and retries after 2 minutes. Official checklist: low inlet gas pressure (if LGPS used), plugged condensate drain, pressure switch wiring, water or sag in the vent piping, a stuck-open low-heat switch, disconnected or obstructed pressure tubing.

On a Carrier 59MN7A, code 35 means the modulating gas valve did not answer a command from the furnace control, or power to the valve electronics was interrupted. The control shuts the furnace down, waits 2 minutes, and restarts the heating cycle.

Official: check for 115VAC at L1/L2 and 24VAC at SEC-1/SEC-2. No light means the board isn't powered.

Official: line-voltage polarity is reversed — and the control blocks heating until it's fixed. (If the furnace is twinned, see the twinning-kit instructions.)

1 short + 1 long flash on recall = nothing stored. Codes self-erase after 72 hours (and on RED-LED boards, whenever power is interrupted).

The furnace was powered up in the middle of a call for heat (R-W closed), so the blower runs 90 seconds first. Informational, not a fault.

A limit, draft safeguard, flame rollout, or blocked-vent switch stayed open longer than 3 minutes → lockout. Auto-resets after 3 hours. Root causes = see code 33.

The system failed to ignite and prove flame in 4 attempts. Auto-resets after 3 hours. Root causes = see code 34.

Official: mis-wired gas valve, a stuck-closed gas-valve relay on the control, or a shorted gas-valve wire. This one does NOT auto-reset.

Flame is proved while the gas valve is de-energized; the inducer runs until the fault clears. Official causes: a leaky or stuck-open gas valve.

Official: obstructed pressure tubing or a pressure switch stuck closed.

A short in the 24VAC wiring blew the board's automotive-type 3-amp fuse. Official: replace with a 3-amp fuse ONLY.

If the switch stays open over 5 minutes, the inducer rests 15 minutes and retries. Official checklist: excessive wind, low inlet gas pressure (if LGPS used), disconnected or obstructed pressure tubing, defective switch.

A limit, draft safeguard, flame rollout, or blocked-vent switch is open. The blower runs 4 minutes or until it closes; open >3 minutes escalates to lockout code 13. Rollout and BVSS switches need a manual reset.

No flame proved during the trial for ignition; the control retries 3 more times before lockout 14. Official checklist: flame sensor must not be grounded, oxide buildup (clean with fine steel wool), proper microamps (0.5 min, 4.0–6.0 nominal), gas valve on, igniter health, inlet gas pressure.

Auto-resets after 1 hour. Official causes: gas-valve relay stuck open, flame-sense circuit failure, or a software check error. Reset power to clear; replace the control if it repeats.

Official: check for 115VAC at L1/L2 and 24VAC at SEC-1/SEC-2. No light means the board isn't powered.

Official: line-voltage polarity is reversed — and the control blocks heating until it's fixed. (If the furnace is twinned, see the twinning-kit instructions.)

1 short + 1 long flash on recall = nothing stored. Codes self-erase after 72 hours (and on RED-LED boards, whenever power is interrupted).

The furnace was powered up in the middle of a call for heat (R-W closed), so the blower runs 90 seconds first. Informational, not a fault.

A limit, draft safeguard, flame rollout, or blocked-vent switch stayed open longer than 3 minutes → lockout. Auto-resets after 3 hours. Root causes = see code 33.

The system failed to ignite and prove flame in 4 attempts. Auto-resets after 3 hours. Root causes = see code 34.

Official: mis-wired gas valve, a stuck-closed gas-valve relay on the control, or a shorted gas-valve wire. This one does NOT auto-reset.

Flame is proved while the gas valve is de-energized; the inducer runs until the fault clears. Official causes: a leaky or stuck-open gas valve.

Official: obstructed pressure tubing or a pressure switch stuck closed.

A short in the 24VAC wiring blew the board's automotive-type 3-amp fuse. Official: replace with a 3-amp fuse ONLY.

If the switch stays open over 5 minutes, the inducer rests 15 minutes and retries. Official checklist: excessive wind, low inlet gas pressure (if LGPS used), disconnected or obstructed pressure tubing, defective switch.

A limit, draft safeguard, flame rollout, or blocked-vent switch is open. The blower runs 4 minutes or until it closes; open >3 minutes escalates to lockout code 13. Rollout and BVSS switches need a manual reset.

No flame proved during the trial for ignition; the control retries 3 more times before lockout 14. Official checklist: flame sensor must not be grounded, oxide buildup (clean with fine steel wool), proper microamps (0.5 min, 4.0–6.0 nominal), gas valve on, igniter health, inlet gas pressure.

Auto-resets after 1 hour. Official causes: gas-valve relay stuck open, flame-sense circuit failure, or a software check error. Reset power to clear; replace the control if it repeats.

Code 10 means the control has entered a one-hour soft lockout. Four failed ignitions, five flame losses in one heat call, unexpected flame or a dead-blower detection can cause it, and the reason code alternates with 10.

Code 11 records failed ignition. After four consecutive failed ignition attempts, the display alternates 10 and 11 and the furnace enters a one-hour lockout.

Code 12 means the control is sensing a weak flame signal. The code reports the signal level only; it does not by itself prove the flame sensor is dirty.

Code 13 means the burner lit and the flame was then lost. If this happens five times in one heat call, 10 and 13 alternate and the furnace locks out for one hour.

Code 14 means the control sensed flame when none should be present. 14 and 10 can alternate, and the one-hour lockout timer starts only once the unexpected flame is no longer detected.

Code 20 reports a twinning fault: a problem with the circuit or configuration used when two furnaces are linked to operate together. It is not a burner or flame fault.

Code 22 means the main limit or the HALC is open. The limit protects against excessive outlet air temperature, so the control stops burner operation while it is open.

Code 26 means the control has detected reversed line and neutral polarity on the furnace's power supply. Correct polarity matters because it can affect flame sensing.

Code 33 means a manual-reset limit control, such as a flame-rollout switch, is open. A technician must identify which device opened and why before the furnace runs again.

Code 55 means the pressure switch circuit reads closed when the control expects it open. The contacts must be open before the next heating cycle can begin.

Code 57 means the pressure switch stayed open when the control expected it closed. If it does not close within the 60-second proving period, the furnace enters a five-minute soft lockout.

Code 61 means the control has inferred a dead blower because the main limit stayed open for more than 150 seconds. It is a deduction from the limit circuit, not a direct motor test.

Code 93 is a hard lockout for an internal control fault, which can include abnormal voltage persisting at the gas-valve contacts. Cycling the thermostat heat call does not clear it.

0 is the normal standby display. The control is powered and waiting for a call from the thermostat; it does not indicate a fault.

C means the compressor is running for cooling or heat-pump heating. It is a normal operating display on this control, not a fault.

H means the control is running a gas heating call. It shows the operating mode only and does not by itself confirm that the burner lit or warm air is reaching the rooms.

F means the continuous fan is on. The blower runs without a heating or cooling call, so register air will be close to room temperature.

E01 means the integrated furnace control (IFC) has detected an internal failure or an IRQ loss. The fault lies in the control board's own operation.

E2.1 means the furnace made three ignition attempts in one heat call and never sensed flame. The control then enters a one-hour lockout.

E2.2 means flame was established and then lost, and this repeated until ten recycles accumulated in one heat call. The control then enters a one-hour lockout.

E2.3 means the control did not detect 24 VAC at the first-stage gas valve (MVL) when it was required, ten times in one heat call. The manual index states this backwards; the main chart and detailed page agree.

E3.1 means the first-stage pressure switch is closed when the control expects it to be open. The manual notes that the switch itself is usually not the root cause.

E3.2 means the first-stage pressure switch is open when the control expects it to be closed, so the required draft has not been proven.

E04 means one of three safety switches is open: the main thermal limit, a flame rollout switch or the reverse-airflow switch. The code does not say which one tripped.

E05 means the control sensed flame at a time when no flame should be present. It is a safety fault, not a normal operating state.

E6.1 means the control detected reversed line-voltage polarity at the furnace power supply. It is an electrical supply fault, not a burner fault.

E6.2 means the control has detected a poor ground connection. It points to the furnace's ground path, which needs electrical testing.

E6.3 means the igniter relay has failed or the igniter is open. The detailed diagnostic page also covers a relay stuck closed and an igniter circuit that is open or shorted.

E7.1 means 24 V is present at the first-stage gas valve circuit when it should be off. The manual index lists this as E07; that is a document inconsistency, not a confirmed alternate code.

E08 means flame current is low, below the 0.5 microamp DC threshold in the detailed diagnostics. The furnace is still allowed to operate; this is not a lockout.

On the S8X1, E11 means the first-stage gas valve was not energized when it should have been, or the first-stage valve relay is stuck closed. Second-stage conditions in the chart do not apply.

E12 means the 5 A fuse on the control board is open or missing. An open fuse suggests something in the low-voltage circuit drew too much current.

IdL means idle: there is no cooling, heating or fan demand from the thermostat. The status reflects thermostat inputs, so it is normal when nothing is being called.

Ht means the control sees a gas-heat demand from the thermostat. It reflects the 24 VAC input only and does not confirm the burner lit or that heat is reaching the rooms.

CP1 means the thermostat is calling for first-stage cooling or heat-pump operation. It is a normal demand display, not a furnace fault.

CP2 means the thermostat is calling for second-stage cooling or heat-pump operation. On the S8X1 this is a normal demand display, not a furnace fault.

COf means the thermostat is calling for continuous fan. The blower runs without a heating or cooling call, so register air stays near room temperature.

dFt means the outdoor heat pump has a defrost demand and the furnace is in gas-heat mode during that period. It is a normal demand state, not a fault.

E01 means the integrated furnace control (IFC) has detected an internal failure or an IRQ loss. The fault lies in the control board's own operation.

E2.1 means the furnace made three ignition attempts in one heat call and never sensed flame. The control then enters a one-hour lockout.

E2.2 means flame was established and then lost, and this repeated until ten recycles accumulated in one heat call. The control then enters a one-hour lockout.

E2.3 means the control did not detect 24 VAC at the first-stage gas valve (MVL) when it was required, ten times in one heat call. The manual index states this backwards; the main chart and detailed page agree.

E3.1 means the first-stage pressure switch is closed when the control expects it to be open. The manual notes that the switch itself is usually not the root cause.

E3.2 means the first-stage pressure switch is open when the control expects it to be closed, so the required draft has not been proven.

E04 means one of three safety switches is open: the main thermal limit, a flame rollout switch or the reverse-airflow switch. The code does not say which one tripped.

E05 means the control sensed flame at a time when no flame should be present. It is a safety fault, not a normal operating state.

E6.1 means the control detected reversed line-voltage polarity at the furnace power supply. It is an electrical supply fault, not a burner fault.

E6.2 means the control has detected a poor ground connection. It points to the furnace's ground path, which needs electrical testing.

E6.3 means the igniter relay has failed or the igniter is open. The detailed diagnostic page also covers a relay stuck closed and an igniter circuit that is open or shorted.

E7.1 means 24 V is present at the first-stage gas valve circuit when it should be off. The manual index lists this as E07; that is a document inconsistency, not a confirmed alternate code.

E08 means flame current is low, below the 0.5 microamp DC threshold in the detailed diagnostics. The furnace is still allowed to operate; this is not a lockout.

On the S8X1, E11 means the first-stage gas valve was not energized when it should have been, or the first-stage valve relay is stuck closed. Second-stage conditions in the chart do not apply.

E12 means the 5 A fuse on the control board is open or missing. An open fuse suggests something in the low-voltage circuit drew too much current.

IdL means idle: there is no cooling, heating or fan demand from the thermostat. The status reflects thermostat inputs, so it is normal when nothing is being called.

Ht means the control sees a gas-heat demand from the thermostat. It reflects the 24 VAC input only and does not confirm the burner lit or that heat is reaching the rooms.

CP1 means the thermostat is calling for first-stage cooling or heat-pump operation. It is a normal demand display, not a furnace fault.

CP2 means the thermostat is calling for second-stage cooling or heat-pump operation. On the S8X1 this is a normal demand display, not a furnace fault.

COf means the thermostat is calling for continuous fan. The blower runs without a heating or cooling call, so register air stays near room temperature.

dFt means the outdoor heat pump has a defrost demand and the furnace is in gas-heat mode during that period. It is a normal demand state, not a fault.

The ignition control has detected that the line-voltage supply is wired with reversed polarity. The control uses this code to report a power-wiring condition, not a failed component.

The ignition control has detected an improper earth ground at the furnace. The code reports the electrical condition the board sees; it does not identify where in the circuit the ground is missing.

During a heat demand, the burners either failed to ignite or the control lost the flame signal after ignition. The same three-flash code covers both events, so it does not tell you which one occurred.

The control is reading a weak flame signal. Lennox's table pairs this code with a flame-sensor check and does not list it as a lockout.

The control reached its maximum number of ignition retries or recycles and entered Watchguard. Lennox service literature describes five total ignition trials followed by a 60-minute Watchguard period before the control tries again.

The primary or secondary limit switch has opened. When the primary limit opens, the gas valve shuts and the blower runs; the same code also marks Watchguard if a limit stays open longer than 3 minutes.

The flame-rollout safety switch is open. The control closes the gas valve, and the switch requires a manual reset after a technician identifies and corrects the cause.

During a heat demand, the pressure switch either did not close or opened after closing. It is part of the pressure-proving circuit tied to the combustion-air inducer and venting.

The pressure switch opened five times during a single heat demand, so the control entered Watchguard. It reflects repeated loss of pressure proving rather than one isolated event.

The pressure switch was already closed before the combustion-air inducer started. The control expects it open at that point, so the reading points to a switch or control-circuit condition.

The control sensed flame while the gas valve was not energized. Flame should not be present then, so this is a safety condition, not a routine fault.

The ignition control is reporting low line voltage on the furnace's power supply. The code identifies a supply condition, not a specific failed part in the furnace.

A dark LED means the control has no power or has detected a board fault. It does not say which of the two applies.

This is a normal status. The control is idle with no heat demand, running the fan continuously, or operating in cooling mode.

The control is reporting an active heat call and burner operation sequence. Lennox's 103217-03 table does not give exact on/off timing for this flash.

The control sensed flame when there was no call for heat. In response it runs the combustion-air inducer and the supply-air blower.

At the start of a heat cycle the pressure switch was already closed when the control expected it open. The control blocks the ignition sequence while this condition exists.

The control expected the pressure switch to be closed and found it open. York lists several checks for this, so the code alone does not prove a failed switch.

The main limit switch is open, and the supply blower and inducer keep running while it stays open. Five openings in one heat demand cause a 1-hour soft lockout; if it stays open past 5 minutes, the code changes to 11 red.

The manual-reset rollout safety switch has opened. The control goes into a hard lockout that York permits clearing only after the problem is corrected.

The pressure switch opened four times during one heat call. The control then locks out for one hour before an automatic retry.

The furnace made three ignition trials without establishing flame. The control then locks out for one hour and retries automatically.

Flame was lost five times during heating, which corresponds to four recycles. The control locks out for one hour, then retries automatically.

The control has detected reversed line-voltage polarity, a grounding problem, or reversed low-voltage transformer wires. Heating and cooling are both affected, and ignition is blocked until it is corrected.

The control detects the gas valve energized when there is no call for heat. The inducer and main blower run, and no ignition sequence starts while it persists.

The main limit has stayed open for more than 5 minutes, putting the control in a hard lockout. York says the lockout should be cleared only after the problem is corrected.

The control received a thermostat Y signal without a G signal. York states heating and cooling still operate normally; the indication flags a wiring issue and is unrelated to the four-red limit fault.

The furnace control is operating normally with no thermostat calls. This is the standby state, not a fault code.

The control is operating normally with a heat call present. This slow amber flash is a status, not a one-flash fault, and does not by itself confirm the burner is lit.

No LED is displayed. York lists lost control power, a blown fuse or a control failure as possible reasons, so a dark LED alone does not identify a failed board.

An LED lit steadily in red, green or amber indicates a control failure. York states the control is not field repairable.

A dark amber LED means the control is not showing that it has power, so the power supply or the fuse needs checking. A dark light alone cannot show which one.

A heartbeat, where the amber LED alternates between bright and dim, means the control has low-voltage power. It is a status display, not a counted fault code.

On this control, an amber LED that stays on without flashing means a temporary control lockout. The control has its own one-hour automatic recovery; that is built-in behavior, not a step for you to repeat.

Two amber flashes mean the pressure switch feedback is still closed at a point where the control expects it to be open. The control will not continue the heating sequence on that signal.

Three amber flashes mean the low-stage pressure switch or its relay did not close, or closed and then reopened. If it stays open more than five minutes, the control stops the inducer for 15 minutes before trying again automatically.

Four amber flashes report an open temperature or vent safety circuit, or high-heat-only operation after two successive low-heat limit trips. The circuit can include the main limit, draft safeguard, rollout or blocked-vent switch. When a switch is open, the blower runs four minutes or until it closes, whichever is longer.

Five amber flashes mean the control senses flame when it is not calling for gas. The inducer keeps running until the condition clears.

Six amber flashes mean an ignition trial ended without the control proving flame. The control can make up to three more attempts on its own, and if flame signal is lost the blower may run its off-delay.

Seven amber flashes mean a protection circuit has locked out heating. It follows a circuit open for more than three minutes or ten consecutive high-heat limit trips. The control recovers automatically after three hours.

Eight amber flashes mean the gas control circuit is in a lockout that does not clear on its own. Unlike the timed lockouts on this control, there is no automatic recovery.

Nine amber flashes mean the high-stage pressure switch or its relay did not close, or closed and then reopened.

Ten short amber flashes mean the incoming line-voltage polarity is incorrect. Where two furnaces are twinned, 24-volt phasing is also involved. The code is ten short flashes, not a 1 followed by 0.

This status means power (115 V or 24 V) returned during a heat call, or the heat call ended during the blower-on delay. The blower runs for 90 seconds.

For two heating cycles in a row, the blower did not reach 250 RPM or did not communicate within 30 seconds of being energized, so heating is locked out. It recovers automatically after three hours.

If this shows four times at power-up and stops, the model plug is missing but the control is using stored model data. If it keeps repeating, the model data, plug, switch settings or twinning setup are invalid.

The blower did not reach 250 RPM, or did not communicate, within 30 seconds of being energized or within 10 seconds during steady operation. It differs from 1+5, the lockout after repeated failures.

After four ignition trials without lighting and proving flame, the control has locked out heating. It recovers automatically after three hours. Read it as six short and one long, not seven.