What causes Carrier Gas Furnace 31?

Restricted or blocked vent or combustion-air pipe, including frost or ice build-up on the combustion-air inlet. Excessive wind at the vent termination. Disconnected, cracked, or obstructed pressure switch tubing. Plugged condensate drain, clogged drain trap, or water in a sagging vent pipe (condensing furnaces). Reversed vent and combustion-air pipes, or other moist air entering the combustion-air pipe. Defective inducer motor, a damaged or loose inducer wheel, or low inducer voltage (115 VAC). Improper vent sizing or inadequate combustion air supply. Defective or out-of-calibration pressure switch, or a high-altitude install without the high-altitude pressure switch kit. Low inlet gas pressure on furnaces with a low gas pressure switch (LGPS). On two-stage models: an open wire or bad terminal on the pressure switch wiring, a mis-wired gas valve, or a defective relay on the furnace control.

How do I fix Carrier Gas Furnace 31?

Confirm the code: look through the blower door sight glass and count 3 short flashes then 1 long flash. Write it down before cutting power, since a power cycle can clear the code history. Go outside and check the furnace intake and exhaust pipes (on high-efficiency furnaces, the two plastic pipes) for snow, ice, frost, leaves, or nests. Clear loose debris from the openings only; don't take vent pipes apart. On a condensing furnace, check the condensate drain line and trap for clogs or standing water, and look for a vent pipe that sags and could hold water. With power off at the furnace switch, remove the burner door and check that the small rubber tubes running to the pressure switch are firmly attached and not cracked, kinked, or full of water. Restore power and watch a heating cycle. Listen for the inducer (the small fan that starts first). If it doesn't start, runs slow, or sounds rough, that points to the inducer. If code 31 returns after the vents, drain, and tubing check out, stop and call a pro. Never jumper or bypass the pressure switch: it keeps the burners off when the furnace can't vent safely.

When should I call a pro for Carrier Gas Furnace 31?

The vents and drain are clear but code 31 keeps coming back. The pressure across the heat exchanger needs measuring with a manometer to decide between a bad pressure switch and a draft problem; Carrier's guides replace the pressure switch assembly only when the pressure is adequate. The inducer motor or wheel needs testing or replacing, or the inducer isn't turning in the right direction. Vent sizing, combustion air, or a high-altitude installation needs evaluating (Carrier calls for a high-altitude pressure switch kit above certain elevations). Pressure switch wiring, the furnace control relay, or gas valve wiring needs checking with a meter. You smell gas or suspect carbon monoxide: leave the house and call the gas utility or 911.

Code: 31🔥 Carrier Gas Furnace

Carrier Furnace Code 31: Pressure Switch Did Not Close or Reopened (Meaning, Causes, Fixes)

Q: What does Carrier Gas Furnace 31 mean?

Status code 31 on a Carrier furnace (3 short flashes, then 1 long flash) means a pressure switch that proves the vent is drafting did not close, or opened again during the heating cycle. On single-stage furnaces it is the main pressure switch; on two-stage and variable-speed furnaces it is the high-heat pressure switch (or its relay). Blocked or icy vent pipes, a water-logged drain, disconnected pressure tubing, wind, and a weak inducer are the usual causes. Carrier status codes are two-digit numbers where the first digit is the number of short flashes and the second the number of long flashes, so code 31 is 3 short flashes followed by 1 long flash. On single-stage Carrier furnaces (for example the 58MCB and 58PHA/PHX service instructions), code 31 reads "Pressure switch did not close or reopened": if the switch stays open longer than five minutes, the inducer shuts off for 15 minutes before it retries, and if it opens during the blower on-delay the blower comes on for the selected off-delay. On two-stage and variable-speed models (58MVP, 58MVB, 58UVB and other 2-stage controls), code 31 means "High-heat pressure switch or relay did not close or reopened": the high-heat pressure switch didn't close on a call for high heat, or opened during high heat. On the variable-speed control, if the switch doesn't make within 45 seconds the furnace shuts down, waits 2 minutes and restarts, and after some faults the next cycle is restricted to high heat. Carrier's guides note that a defective control relay or a mis-wired gas valve can also cause it. The pressure switch is a safety device: it keeps the burners off unless the inducer is pulling enough draft through the heat exchanger and vent.

Reviewed 2026-10-23 · Based on 4 manufacturer & service sources

What the code means

Carrier status codes are two-digit numbers where the first digit is the number of short flashes and the second the number of long flashes, so code 31 is 3 short flashes followed by 1 long flash. On single-stage Carrier furnaces (for example the 58MCB and 58PHA/PHX service instructions), code 31 reads "Pressure switch did not close or reopened": if the switch stays open longer than five minutes, the inducer shuts off for 15 minutes before it retries, and if it opens during the blower on-delay the blower comes on for the selected off-delay. On two-stage and variable-speed models (58MVP, 58MVB, 58UVB and other 2-stage controls), code 31 means "High-heat pressure switch or relay did not close or reopened": the high-heat pressure switch didn't close on a call for high heat, or opened during high heat. On the variable-speed control, if the switch doesn't make within 45 seconds the furnace shuts down, waits 2 minutes and restarts, and after some faults the next cycle is restricted to high heat. Carrier's guides note that a defective control relay or a mis-wired gas valve can also cause it. The pressure switch is a safety device: it keeps the burners off unless the inducer is pulling enough draft through the heat exchanger and vent.

Likely causes

Restricted or blocked vent or combustion-air pipe, including frost or ice build-up on the combustion-air inlet
Excessive wind at the vent termination
Disconnected, cracked, or obstructed pressure switch tubing
Plugged condensate drain, clogged drain trap, or water in a sagging vent pipe (condensing furnaces)
Reversed vent and combustion-air pipes, or other moist air entering the combustion-air pipe
Defective inducer motor, a damaged or loose inducer wheel, or low inducer voltage (115 VAC)
Improper vent sizing or inadequate combustion air supply
Defective or out-of-calibration pressure switch, or a high-altitude install without the high-altitude pressure switch kit
Low inlet gas pressure on furnaces with a low gas pressure switch (LGPS)
On two-stage models: an open wire or bad terminal on the pressure switch wiring, a mis-wired gas valve, or a defective relay on the furnace control

Safe DIY checks

⚠ Before you start

Turn off power at the furnace/HVAC service switch and the breaker before opening panels. Shut the gas valve before any work near the burners, and never disconnect or alter the gas line. If you smell gas, leave the house and call your gas utility.

Confirm the code: look through the blower door sight glass and count 3 short flashes then 1 long flash. Write it down before cutting power, since a power cycle can clear the code history.
Go outside and check the furnace intake and exhaust pipes (on high-efficiency furnaces, the two plastic pipes) for snow, ice, frost, leaves, or nests. Clear loose debris from the openings only; don't take vent pipes apart.
On a condensing furnace, check the condensate drain line and trap for clogs or standing water, and look for a vent pipe that sags and could hold water.
With power off at the furnace switch, remove the burner door and check that the small rubber tubes running to the pressure switch are firmly attached and not cracked, kinked, or full of water.
Restore power and watch a heating cycle. Listen for the inducer (the small fan that starts first). If it doesn't start, runs slow, or sounds rough, that points to the inducer.
If code 31 returns after the vents, drain, and tubing check out, stop and call a pro. Never jumper or bypass the pressure switch: it keeps the burners off when the furnace can't vent safely.

🛠 When to call a pro

The vents and drain are clear but code 31 keeps coming back.
The pressure across the heat exchanger needs measuring with a manometer to decide between a bad pressure switch and a draft problem; Carrier's guides replace the pressure switch assembly only when the pressure is adequate.
The inducer motor or wheel needs testing or replacing, or the inducer isn't turning in the right direction.
Vent sizing, combustion air, or a high-altitude installation needs evaluating (Carrier calls for a high-altitude pressure switch kit above certain elevations).
Pressure switch wiring, the furnace control relay, or gas valve wiring needs checking with a meter.
You smell gas or suspect carbon monoxide: leave the house and call the gas utility or 911.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Pressure switch hose / tubing

Cheap fix for cracked, brittle, or disconnected tubing.

[affiliate link slot: AFFILIATE_SLOT_PRESSURE_HOSE]

⚙

Pressure switch (exact OEM part number for your model)

Match the part number on your existing switch; the rating matters. Replace only after a pro confirms the draft is adequate.

[affiliate link slot: AFFILIATE_SLOT_PRESSURE_SWITCH]

⚙

Induced draft blower (inducer motor) assembly

Model-specific; usually a pro install.

[affiliate link slot: AFFILIATE_SLOT_INDUCER]

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Digital multimeter

For checking 24 VAC at the pressure switch wires and inducer voltage.

[affiliate link slot: AFFILIATE_SLOT_MULTIMETER]

Sources

Last reviewed 2026-10-23. Always confirm against the manual for your exact model number.