Carrier furnace Code 31 — Pressure switch did not close (or reopened)

What this error means

The draft pressure switch never proved airflow through the vent system, so the furnace refuses to ignite. Per Carrier's service documentation, if it stays open more than 5 minutes the inducer shuts off for 15 minutes before retrying. Usual suspects: a blocked or restricted vent (check the outside termination for snow, ice, leaves, or nests), a weak draft inducer motor, disconnected pressure tubing, or — on high-efficiency condensing furnaces — a clogged condensate trap holding the switch open.

Symptoms

• Furnace won't ignite; inducer fan may run continuously
• Code 31 blinked on the status LED
• More common in winter (blocked vent termination) or on 90%+ furnaces with drain issues

Try this first (safe DIY steps)

1. Check the exterior vent/intake pipes for blockage (snow, ice, debris, nests) and clear them
2. On a high-efficiency furnace, check the condensate drain/trap isn't clogged or frozen
3. Power off at the furnace switch for 30 seconds, restore, and watch whether the code returns
4. Note: on two-stage models, Code 31 is specifically the HIGH-heat pressure switch (32 is low-heat)

Safety: power off before any physical intervention. Never bypass covers or interlocks — fuser areas run hot enough to burn.

When to call a technician

If venting and condensate are clear and 31 returns — pressure switch replacement runs $150–$400; a failing inducer motor is $400–$1,100 installed.