Carrier Furnace Error Code 31 – Meaning, Causes & Fix

What Does 31 Mean?

What Causes This Error?

• Blocked or frozen condensate drain line backing up into the pressure switch hose
• Cracked, kinked, or disconnected pressure switch rubber hose
• Inducer (draft) motor failing or not reaching operating speed
• Pressure switch failed in the open position
• Blocked flue exhaust or combustion air intake pipe (birds nest, ice, debris)
• Extremely dirty air filter restricting combustion airflow
• Cracked heat exchanger allowing flue gases to affect pressure readings
• Control board relay failure preventing pressure switch circuit from reading correctly

How to Fix 31

1. 1

   Read the blink code to confirm error 31. Look through the small sight glass window on the furnace access panel. Count the LED blink pattern: 3 blinks, pause, 1 blink. This confirms error code 31 — a primary limit/pressure switch fault. Note: on some Carrier models the diagnostic LED chart is printed on the inside of the access panel door. Confirm the code before proceeding.

2. 2

   Check and clear the condensate drain. On a high-efficiency (90%+) furnace, locate the condensate drain line — typically a white PVC pipe exiting the furnace and running to a floor drain or condensate pump. Check for blockages: pour a cup of water into the condensate trap and observe whether it drains freely. If it backs up, the trap or drain line is blocked. Use a wet/dry vacuum to suction the drain line clear from the outlet end. A blocked condensate drain is the single most common cause of code 31 on Carrier 90% furnaces.

3. 3

   Inspect the pressure switch hose. With the furnace powered off (turn off at the thermostat and flip the furnace power switch), open the access panel and locate the small rubber or vinyl hose(s) running from the inducer housing to the pressure switch(es). Check each hose for cracks, kinks, or loose connections at either end. Reconnect any loose hose firmly. If a hose is cracked, it must be replaced — use the same diameter vinyl tubing available at hardware stores.

4. 4

   Inspect the flue exhaust and combustion air intake pipes. Go outside and visually inspect the PVC flue pipes exiting your home. Check for bird nests, ice blockages, debris, or crushed pipe sections. On a two-pipe system, both the exhaust and intake must be clear. Remove any obstructions. In cold climates, ice can form at the pipe exit during very cold weather — carefully remove ice without damaging the pipe.

5. 5

   Replace the air filter. Locate the furnace air filter (in the return air duct or inside the furnace cabinet). Check its condition — a filter that is gray or brown and visibly clogged must be replaced. A severely restricted filter reduces airflow through the heat exchanger, affecting combustion and inducer draft. Insert a new filter of the same size and MERV rating.

6. 6

   Test the inducer motor. With the furnace powered on, initiate a heat call from the thermostat. Listen for the inducer motor starting — it should come on within 30 seconds and run steadily before the ignitor glows. If you hear grinding, humming without rotation, or no sound at all, the inducer motor is failing. Do not attempt inducer motor replacement without a licensed HVAC technician.

When to Call a Professional

Can You Still Use the Device?

Do not attempt to run the furnace while error code 31 is active. The safety lockout is preventing gas valve operation specifically because safe combustion conditions cannot be confirmed. Running the furnace with a defeated pressure switch is a carbon monoxide hazard. Resolve the fault before resuming heating.