Carrier 58MVC Furnace Error Code 42: Inducer Motor Fault
What Does Code 42 Mean?
The inducer (draft) motor on the 58MVC pulls combustion gases through the heat exchanger and out the vent, and it must reach a valid speed before the burners are allowed to fire. Code 42 is set when the inducer does not start within 20 seconds of a call for heat, when its RPM falls outside the valid operating range, or when the RPM signal is lost for 5 seconds during operation.
On this condensing, sealed-combustion furnace, the inducer draws combustion air through a dedicated intake pipe and exhausts through a second pipe. The manual lists a failed inducer motor, improper motor wiring, and a restricted combustion-air supply or incorrectly sized vent as the causes of this fault. All three require hands-on diagnosis: confirming the vent and intake are sized and routed to the manufacturer's specification is an installation check, testing whether the inducer spins freely and holds its target RPM requires test equipment, and correcting wiring is electrical work.
The most common cause found in the field is a failing inducer motor that cannot reach or hold speed, which the board detects through the motor's speed feedback signal. Unlike the pressure-switch codes (23, 31, 32, 43), which report on draft *proving* through a separate switch, code 42 reports on the inducer motor and its supporting wiring and venting directly.
Diagnosing and correcting any of these — the motor, its wiring, or the combustion-air venting — is professional work; there is no homeowner-safe fix for this code.
What You'll Notice
- The furnace is silent on a call for heat — you never hear the inducer spin up
- The inducer starts but sounds slow, straining, rattling, or unusually loud
- The burners never light because the draft was never proven
- No heat and no other codes that point to airflow or ignition
- The outside intake or exhaust pipe appears blocked or damaged by snow, ice, or debris
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Failed inducer motor | Most common | ✗ Call a pro → |
| Restricted combustion air supply | Common | ✗ Call a pro → |
| Improper motor wiring | Uncommon | ✗ Call a pro → |
How This Is Diagnosed
A technician starts by confirming the combustion-air intake and vent terminations are unobstructed and sized and routed correctly per the installation manual, since restricted venting is one of the manual's listed causes. They then check whether the inducer motor spins freely, test the motor and its capacitor if fitted, verify the RPM feedback wiring, and measure whether the motor reaches and holds its target speed. Correcting vent sizing, replacing the inducer motor, or repairing its wiring is professional work.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- The furnace shows code 42 — diagnosing whether the cause is the inducer motor, its wiring, or the venting requires a technician
- You never hear the inducer start, or it sounds slow, grinding, or rattling
- A visible vent pipe section is crushed, disconnected, or damaged
- The furnace was recently installed or serviced and code 42 appeared afterward, which can point to vent sizing or routing
Frequently Asked Questions
What is the inducer motor on a Carrier 58MVC?
It is the small motor and fan that pulls combustion gases through the heat exchanger and pushes them out the vent. The furnace will not fire until this motor reaches a valid speed, which is why a fault stops ignition.
Can I fix code 42 myself?
No. The manual's causes for this code — a failed inducer motor, incorrect motor wiring, or a restricted or improperly sized combustion-air vent — all require a technician's tools and knowledge of the installation specification to diagnose and correct.
Why won't my furnace start at all with code 42?
The control requires proven inducer draft before it opens the gas valve. If the inducer does not start or reach its target speed, the board never allows ignition, so the furnace stays silent.
How much does an inducer motor replacement cost?
It varies significantly by region and model, and the motor is a mid-range furnace part. A technician can quote the exact cost after confirming the motor is the failure.
✓ Verified against manufacturer service manual — March 2026