CARRIER DIAGNOSTIC REFERENCE STATUS LED — TWO-STAGE VARIABLE-SPEED (58MVP / 58MVB / 58UVB) DOC CARRIER-43

Carrier furnace: Code 43 — Low-Heat Pressure Switch Open in High Heat

Q: What does code 43 mean on a Carrier furnace?

Code 43 — Low-Heat Pressure Switch Open in High Heat. 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.

YOUR LIGHT: 4 SHORT + 3 LONG FLASHES — MATCH IT ABOVE

Try DIY first, then call a pro

What does code 43 mean on a Carrier furnace? 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.

GOOD NEWS: THE FIRST CHECKS ARE FREE AND TAKE MINUTES.

Try these first — in this order

Drain trap, vent pipe slope, and the pressure tubing to both switches.

1Check: Plugged condensate drain, or water pooling in a sagging vent pipe

2Check: Disconnected or obstructed pressure tubing

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Switch testing and a wiring check, since the two switches are easy to cross. DIY part cost: $30–60 — a pro visit for this class of fault typically runs $150–350.

What causes it (in order of likelihood)

Plugged condensate drain, or water pooling in a sagging vent pipe DIY-CHECKABLE
Disconnected or obstructed pressure tubing DIY-CHECKABLE
Low-heat pressure switch stuck open, or the two switches miswired TECH
Low inlet gas pressure (if LGPS used) TECH

Parts referenced by this code

Pressure Switch

$30–60 · Model-specific

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Always match parts against your model's parts list — part numbers vary within a series.

Applies to

58MVP (Series 170/180) · 58MVB · 58UVB

Different Carrier generations use different code maps. If your series isn't listed, don't assume this meaning — check your blower-door label.

Verified against Carrier 58MVP/58MVB/58UVB Troubleshooting Guide (58-06T), p.8/30 — “LOW-HEAT PRESSURE SWITCH OPEN WHILE HIGH-HEAT PRESSURE SWITCH IS CLOSED – Check for: Low inlet gas pressure (if LGPS used). Plugged condensate drain. Improper pressure switch wiring. Water in vent piping, possible sagging pipe. Stuck open low-heat pressure switch. Disconnected or obstructed pressure tubing.”

Verified against Carrier 58MVP/58MVB/58UVB Troubleshooting Guide (58-06T), p.30 — “If the high heat pressure switch HPS makes and the low heat pressure switch LPS is still open the variable speed furnace control starts flashing status code 43, shuts unit down, and waits 2 minutes before restarting the heating cycle.”

Verified against Carrier 58MVB Service and Maintenance Instructions (58MVB-2SM), p.17 — “Check for: - Low-heat pressure switch stuck open. - Disconnected or obstructed pressure tubing. - Improper pressure switch wiring. - Low inlet gas pressure (if LGPS used). - Plugged condensate drain. - Water in vent piping, possible sagging pipe.”

Last verified 2026-09-05 · how we verify

Related Carrier fault codes

These codes use the same status led — two-stage variable-speed (58mvp / 58mvb / 58uvb) system.

Pattern	Meaning
	Code 42 — Inducer Motor Fault
	Code 41 — Blower Motor Fault
	Code 32 — Low-Heat Pressure Switch Did Not Close or Reopened
	Code 25 — Model Selection or Setup Error

See all Carrier furnace fault codes →