Carrier Error Code 31 — What It Means
Error code 31 on a Carrier furnace indicates a pressure switch fault. The furnace’s low-pressure or high-pressure switch either did not close when the control board expected it to during startup, or it opened unexpectedly while the burners were running. The pressure switch is a safety device that monitors the draft created by the inducer motor. If the switch does not sense adequate draft (usually caused by blocked venting, a weak inducer, or a plugged condensate drain), it will not allow the burners to light or will shut them down.
This code protects against incomplete combustion and carbon monoxide risk. The most common real-world causes are blocked intake or exhaust vents, water or debris in the pressure switch tubing, a clogged condensate trap, or a failing inducer motor. Less often, the pressure switch itself has failed or there is a wiring problem. A cracked heat exchanger can also disrupt draft pressure, though this is much less common.
Common Causes
- Blocked venting or intake Snow, ice, debris, bird nests, or improper termination can restrict airflow and prevent the inducer from building the draft needed to close the pressure switch.
- Water or blockage in pressure switch tubing Condensate can back up into the small hose that connects the inducer housing to the pressure switch, preventing the switch from sensing draft correctly.
- Clogged condensate trap or drain line A blocked condensate trap or drain causes water to back up into the inducer or pressure tubing, disrupting the draft signal to the switch.
- Weak or failing inducer motor If the inducer runs slowly, makes abnormal noise, or does not create strong enough draft, the pressure switch will not close on startup.
- Failed pressure switch The switch itself can stick open, fail to close under correct draft, or develop internal faults that prevent proper signaling to the control board.
- Wiring or control board issue Loose connections, corroded terminals, or a faulty board can prevent the pressure switch signal from being recognized even when the switch closes properly.
Step-by-Step Fix
- Confirm the error code by watching the furnace LED or display through a full startup attempt and verify that code 31 appears consistently.
- Inspect the pressure switch tubing for kinks, disconnections, cracks, or water inside the hose that runs from the inducer housing or draft assembly to the pressure switch.
- Check the exhaust vent and intake pipe outside the home for blockages such as ice, snow, debris, or animal nests, and verify that both terminations are clear and properly installed.
- Inspect and clean the condensate trap and drain line by removing the trap, flushing it with water, and confirming that condensate drains freely without backup into the inducer.
- Test the inducer motor by listening for smooth, strong operation when the furnace calls for heat, and feel for strong airflow at the draft outlet or intake if accessible.
- Test the pressure switch with a multimeter to confirm it opens and closes correctly, or use a manometer to measure draft pressure and compare to the switch’s rated actuation value if available in your model’s service manual.
- Check wiring and connections at the pressure switch, control board, and inducer, looking for loose spade terminals, corrosion, or damaged wires that could interrupt the signal.
Parts Often Needed
| Part | Notes |
|---|---|
| Pressure switch | Amazon | Match the switch part number and pressure rating to your furnace model exactly. |
| Pressure switch tubing | Amazon | Use clear or translucent tubing so you can inspect for water or blockage in the future. |
| Inducer motor assembly | Amazon | Choose OEM or direct-fit replacement for your Carrier model and serial number. |
When to Call a Pro
Call a licensed HVAC technician if you have cleared all vents and drains, tested the pressure switch tubing, and the code persists. Diagnosing weak draft, measuring actual switch actuation pressure with a manometer, and testing for a cracked heat exchanger require professional tools and training. Also call a pro if you are uncomfortable working with gas appliances, electrical components, or disassembling the inducer and vent system. If the furnace is under warranty, professional documentation of the fault will be required for any covered parts.