Carrier AC / Heat Pump Error 31 – Causes & Fix
The inducer couldn't prove a clear vent, so the pressure switch stayed open and the burners won't light.
Safety first · Consult a professional
Your part is limited to the outdoor vent pipes, the condensate drain and a single reset – never jumper, bypass or adjust the pressure switch, because it is what keeps flue gas from spilling into the house. If you smell gas, get out right away without touching light switches or using a phone inside, and call 911 or the gas utility once you're outdoors; if the CO alarm goes off, evacuate, call 911 and leave the furnace off until a technician has checked it.
- Gas appliance: no DIY repairs. If you smell gas, leave the building, do not use switches and call your gas utility’s emergency line from outside.
- Carbon monoxide is odorless. Keep a CO alarm near fuel-burning appliances; if it sounds, get everyone outside and call 911.
- Unplug the appliance or switch off the breaker before opening any panel. Mains voltage can kill.
At a glance
Severity: Moderate
- Meaning
- Pressure Switch Did Not Close (Code 31)
- Typical repair cost
- $0–$900
- DIY difficulty
- Easy
Before a Carrier gas furnace lets its igniter heat up, the draft inducer motor spins up to pull combustion air through the heat exchanger and push flue gas out the vent. A pressure switch, connected to the inducer housing by a thin rubber hose, has to sense that suction and close before the sequence can continue. Code 31 – three short flashes and one long flash on the amber status LED – means the switch never closed or dropped open during the cycle. On single-stage models this is the only pressure switch; on two-stage and variable-speed furnaces 31 points at the high-heat switch (or its relay), while the low-heat switch reports as 32. If the switch stays open for more than 5 minutes, the control stops the inducer for 15 minutes and then tries again. On 90%+ condensing furnaces the usual culprit is a blocked PVC termination outside or water backing up from a clogged condensate trap; on 80% furnaces a restricted chimney or a tired inducer motor is more likely.
How it looks on the display
Model families & display variants
| Model family | Shows | Note |
|---|---|---|
| Single-stage furnaces such as the 58STA and 59SC5 types | 31 (3 short, 1 long) | Only one pressure switch is used, so 31 covers any failure to prove draft. |
| Two-stage and variable-speed furnaces | 31 or 32 | 31 refers to the high-heat pressure switch or relay; the low-heat switch has its own code 32. |
| Infinity and Evolution communicating systems | Text on the wall control | The wall control spells out the pressure switch fault, and the furnace board still flashes the matching code. |
| Bryant and Payne furnaces | 31 | They use the same Carrier-family controls, so the flash pattern and meaning are identical. |
Symptoms
- Amber LED behind the blower-door sight glass repeats 3 short flashes, then 1 long
- Inducer motor starts and runs, but the igniter never glows and no flame appears
- Furnace goes quiet for roughly a quarter of an hour, then makes another attempt
- Gurgling or sloshing from the inducer area on a high-efficiency furnace
- Problem started during a snowstorm, a hard freeze or strong gusty wind
- Infinity or Evolution wall control shows a pressure switch message
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Intake or exhaust termination outside blocked by snow, ice, leaves or a bird or wasp nest (condensing models)
- Clogged or frozen condensate trap or drain line, so water backs up into the collector box and inducer housing
- Pressure switch hose cracked, loose, kinked or holding water
- Weak inducer motor – worn bearings, slow start or low 115 V supply
- Restricted, undersized or poorly sloped vent pipe, or a blocked chimney on an 80% furnace
- Pressure switch itself defective
- Strong wind across the termination, too little combustion air, or an open low gas-pressure switch where fitted
Fix it step by step
Count the flashes before you open anything
Look through the small window in the blower access door and count: three quick blinks followed by one slow one is code 31. Pulling that door off opens the interlock switch and kills power to the board, so write the code down first.
Tools: Flashlight
Walk outside to the two PVC pipes
On a condensing furnace, find the intake and exhaust terminations on the side wall or roof and clear away drifted snow, ice, leaves or nesting material by hand without pulling the pipes apart. Keep the area below the pipes shoveled clear through the rest of the winter, because a drift can bury them again overnight.
Tools: Flashlight, Gloves
Trace the condensate drain and trap
Follow the small drain tube from the furnace to the floor drain or condensate pump and check that water is actually leaving. A pump that doesn't run, a line lying full of standing water, or a tube running through an unheated garage or crawlspace that has frozen will all back water up into the inducer. Clean or thaw only the external trap and line in the way your owner's manual describes, with a towel and bucket ready.
Tools: Towels & shallow pan, Flashlight
Listen to the start-up with the door on
Turn the thermostat up and stand next to the furnace: the inducer should reach full speed within a few seconds, followed by a faint click from the pressure switch and the igniter glow. A grinding or slow-starting inducer, water gurgling inside, or an inducer that never starts at all are each worth describing to the technician.
Note when the fault appears
Keep track of whether 31 only shows up on windy days, on the coldest nights, or only when the furnace calls for high heat. That pattern helps the technician decide between a venting problem, a draining problem and a weak inducer much faster.
How to reset 31
- Turn the thermostat down or off
- Switch off the furnace service switch (it looks like a light switch near the unit) or its breaker for 30 seconds, then back on
- Raise the thermostat and watch the LED through the sight glass; if 31 comes back, don't keep resetting – call a technician
Where the parts are
- 1Fan
- 2Temperature sensor
- 3CAN bus
- 4Outdoor unit
- 5Compressor
- 6Engine computer (ECM)
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Clear vent terminations or thaw the condensate line yourself | Yes | $0 |
| Technician diagnostic visit | No | $100–$200 |
| Condensate trap and drain cleaning by a technician | No | $100–$250 |
| Pressure switch or hose replacement | No | $150–$350 |
| Draft inducer motor replacement | No | $400–$900 |
Rough US estimates; technician call-out fees vary by region. Not a quote.
When to call a technician
Call a licensed HVAC technician if 31 returns after you have cleared the terminations, confirmed the drain is flowing and done one reset, or right away if the inducer grinds or water drips from its housing. The technician will measure the draft with a manometer against the rating printed on the switch, check the inducer's supply voltage and inspect the vent run – bypassing the switch is never an acceptable fix.
Frequently asked questions
Why does code 31 almost always show up in winter?
Cold weather brings the typical triggers together: drifting snow and ice around the PVC terminations, condensate lines freezing where they pass through unheated spaces, and long run times that expose a weak inducer. Checking the pipes outside after every storm prevents most of these calls.
Can I replace the pressure switch myself?
It's rarely the switch that failed – it is usually reporting a real draft or drainage problem, so a new switch often changes nothing. The switch and inducer are part of the combustion safety chain and belong to a licensed technician, and a switch should never be jumpered to get heat.
What is the difference between code 31 and code 32?
On two-stage and variable-speed Carrier furnaces, 31 is the high-heat pressure switch and 32 the low-heat one. Single-stage furnaces only have one switch and only use 31.
The furnace works again after clearing the vent – do I still need a service call?
If you found a clear blockage and the furnace completes several full cycles afterwards, probably not. If you found nothing and the code keeps coming back on its own, the cause is intermittent, such as water in the hose or wind, and it should be looked at before the next cold spell.
Sources
- Official Carrier user documentation and support pages (facts only; wording is our own)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy