Carrier furnace code 42: Inducer Motor Fault
Try DIY first, then call a pro
What does code 42 mean on a Carrier furnace? 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.
Before you touch anything: the six signs that mean stop troubleshooting and call a pro →
Homeowner checks for Carrier furnace code 42
This is a limited outdoor observation check, not a complete furnace repair. If you smell gas or a carbon monoxide alarm sounds, leave the home right away without operating any switches, then call the gas utility or 911 from outside. Never bypass a safety switch or reset the furnace over and over.
- Turn the furnace off
Turn off the furnace at the thermostat and its nearby power switch before checking the vent ends. Keep the furnace cabinet closed.
- Find the vent ends from ground level
Locate the furnace intake and exhaust pipe ends where they leave the house. Only check ends you can see and reach while standing on the ground. Do not climb a ladder or go onto the roof.
- Clear loose snow or leaves by hand
If it is safe to stand there, brush away loose surface snow or leaves from around the openings by hand, without tools. Do not reach into the pipes, chip at ice, or remove or modify any pipe, cap, or screen.
- Stop and call if anything else is wrong
Leave the furnace off and call a technician if you find ice, a nest, a damaged pipe, or anything you cannot clear safely by hand. After clearing only loose surface snow or leaves, restore power and observe one heating cycle. If the code returns, turn the furnace off and arrange service.
Inducer RPM and wiring diagnosis. The assembly is replaced as a unit. DIY part cost: $250–500 — a pro visit for this class of fault typically runs $150–350.
What causes it (in order of likelihood)
- Blocked intake or exhaust termination (snow, leaves, nests) restricting combustion air DIY-CHECKABLE
- Failed inducer motor TECH
- Loose or incorrect inducer motor wiring TECH
- Vent sizing TECH
Parts referenced by this code
Always match parts against your model's parts list — part numbers vary within a series.
Questions about this code
What does code 42 mean on a Carrier furnace?
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.
How do I fix Carrier furnace code 42?
From ground level, check the outdoor intake and exhaust ends and clear loose snow or leaves by hand if safe. Inducer wiring and speed diagnosis are technician work.
How much does it cost to fix Carrier furnace code 42?
If a blocked termination was the cause, clearing it may be enough. Otherwise cost depends on whether wiring, vent sizing, or the inducer assembly is involved.
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.8/28-29 — “INDUCER MOTOR FAULT – Indicates the inducer has not started within 20 seconds after a call for heat, the inducer motor RPM is outside its valid range of operation, or the inducer RPM signal was lost for 5 seconds during operation. Check for: Proper vent sizing. Restricted combustion air supply. Failed inducer motor. Improper motor wiring.”
Verified against Carrier 58MVB Service and Maintenance Instructions (58MVB-2SM), p.18 — “Check for: Proper vent sizing. Failed inducer motor. Restricted combustion air supply. Improper motor wiring.”
Verified against Carrier 59MN7A Troubleshooting Guide (59MN7A-01T), p.9
Last verified 2026-09-05 · page updated 2026-10-03 · how we verify
Other Carrier furnace codes
These links include other Carrier model families. Code meanings can change by model and control board, so confirm your model and service manual before using a definition.