Carrier Furnace Error Code 31 usually means the pressure switch did not close or reopened during the heating cycle, often because of a venting, inducer, or condensate drainage problem. The single most likely fix is clearing the intake or exhaust vent and checking the pressure switch hose for blockage, cracks, or moisture.

⚡ Quick Fix

1. Shut off power to the furnace at the switch or breaker, wait 15 to 30 minutes, then restore power and call for heat at the thermostat. This resets the control board and can clear a temporary pressure switch lockout caused by a brief draft issue or voltage glitch. If Error Code 31 returns, move to airflow and vent checks.
2. Inspect the intake and exhaust vent pipes outside the home and remove any snow, leaves, nests, ice, or debris. Then check the pressure switch tubing inside the furnace for water, cracks, kinks, or a loose fit at the switch or inducer housing. This is the most common real-world fix for Carrier Furnace Error Code 31.
3. If the code remains, verify the inducer motor starts properly and the pressure switch closes when the draft motor runs. A weak inducer assembly, restricted condensate trap, damaged wiring harness, or faulty pressure switch can all keep the control board from seeing proper draft. At that point, use a multimeter and stop if you are not comfortable testing live components.

What This Error Means

On a Carrier Furnace, Error Code 31 points to a pressure draft safeguard problem. During a call for heat, the inducer motor must create negative pressure in the heat exchanger and vent system. The pressure switch senses that draft and sends a signal to the PCB or control board that venting is safe, allowing ignition to continue.

If the pressure switch does not close at the right time, or if it closes and then opens again during the cycle, the furnace stops the ignition sequence and stores Error Code 31. This protects the home from unsafe combustion, flue gas spillage, and overheating caused by blocked venting or improper inducer operation.

Technically, the fault can be caused by a blocked intake or exhaust pipe, cracked or water-filled pressure tubing, a restricted condensate trap on a high-efficiency furnace, a weak inducer wheel or motor capacitor on some designs, loose wiring, or a failed pressure switch. The control board is usually not the first suspect, but it can also misread the switch input if there is corrosion, a damaged connector, or a PCB fault.

Common Causes

Blocked intake or exhaust vent

Carrier condensing furnaces depend on clear venting to maintain proper pressure differential. Snow, insect nests, leaves, ice, or sagging vent pipe sections can restrict airflow enough to keep the pressure switch from closing or make it drop out mid-cycle.

Clogged condensate trap or drain line

Many Carrier furnaces produce condensate during operation, and that water must drain freely. If the trap, drain hose, or collector box passages clog, water can back up into the inducer housing or pressure switch hose and interfere with draft sensing.

Faulty or damaged pressure switch tubing

The small rubber or silicone hose between the inducer housing and the pressure switch must be airtight and dry. A split hose, loose connection, or moisture inside the tube can prevent the switch diaphragm from seeing the correct vacuum signal.

Weak inducer motor or inducer wheel problem

If the inducer motor is slow, noisy, or not reaching full speed, it may not create enough negative pressure for the switch to close. A failing motor bearing, obstructed wheel, poor voltage supply, or damaged wiring harness can all cause this condition.

Defective pressure switch or control board circuit

Pressure switches do fail, though less often than vent or drainage components. If the switch contacts stay open despite correct draft, or the control board cannot read the signal due to a PCB trace or connector issue, Carrier Furnace Error Code 31 will persist.

Step-by-Step Fix

Tools you may need: screwdriver, multimeter, flashlight

Safety warning: Disconnect power before opening any panels or touching internal components.

1. Start with a full reset and a visual inspection. Turn the furnace off at the service switch or breaker, wait 15 to 30 minutes, then restore power and set the thermostat to heat. Watch the startup sequence through the sight glass or with the panel removed if your model allows safe observation; the inducer should start before ignition is attempted.
2. Check the vent system at both the furnace and the outdoor termination points. Look for disconnected PVC, sagging pipe that traps water, bird nests, insect screens clogged with lint, ice, or snow blockage. If you find obstruction, clear it completely and make sure the vent and intake are not sharing a blockage source such as drifting snow or yard debris.
3. Inspect the condensate system if your Carrier furnace is a condensing model. Remove and clean the condensate trap if accessible, flush drain tubing, and make sure water can flow freely to the drain. If you see water in the pressure switch hose or inducer port, dry the hose and clear the port carefully because backed-up condensate is a very common trigger for Error Code 31.
4. Examine the pressure switch tubing and wiring harness. Remove the hose and look for cracks, brittleness, soft spots, kinks, or water droplets inside; replace damaged tubing and reseat both ends tightly. Then check the pressure switch electrical terminals and control board connector for corrosion, loose spade terminals, or broken wires.
5. Test inducer operation during a call for heat. When the thermostat demands heat, the inducer motor should start promptly and sound smooth, not strained or intermittent. If it hums, runs slowly, squeals, or stops unexpectedly, the inducer assembly may be failing and should be professionally tested for amperage draw, supply voltage, and mechanical drag.
6. Use a multimeter only if you are comfortable diagnosing electrical components. With power disconnected, verify continuity through the pressure switch only after confirming the switch is being tested correctly for its operating state; under live operation, a technician may also measure whether the switch closes when the inducer creates draft. Do not bypass the pressure switch to force operation, because that defeats an important combustion safety circuit.
7. Check the furnace air filter and return airflow if other steps do not solve the issue. While a dirty filter is not the primary cause of Code 31, severe airflow issues can contribute to abnormal burner operation, condensate production, and repeated lockouts on some systems. Replace an overly restrictive filter and make sure supply and return registers are open.
8. Stop and call a professional if the code returns after vent, drain, tubing, and inducer checks. A technician can measure pressure switch setpoint performance, inspect the collector box and heat exchanger passages, test the control board input circuit, and confirm whether the pressure switch, inducer motor, or PCB needs replacement. Call immediately if you smell gas, see flame rollout, or notice water leaking into electrical components.

Related Errors

• Error 13 — Limit circuit lockout, usually caused by overheating, restricted airflow, or an open high-limit switch.
• Error 14 — Ignition lockout after repeated failed ignition attempts.
• Error 33 — Limit or flame rollout switch open, indicating overheating or a burner compartment safety fault.
• Error 34 — Ignitor proving error or ignition-related control fault on some Carrier furnace families.
• Error 41 — Blower motor fault or blower motor outside valid speed range, depending on furnace design.
• Error 42 — Inducer motor or inducer feedback issue, often tied to voltage, relay, or motor operation problems.

FAQ

Can I reset this error without a technician?

Yes, you can try a basic reset by shutting off power to the furnace for 15 to 30 minutes and then restarting it. If Carrier Furnace Error Code 31 was caused by a temporary draft disturbance, it may clear. If the code comes back, the furnace still has a real venting, condensate, switch, or inducer problem that needs diagnosis.

Is Carrier Furnace Error Code 31 dangerous?

It can be, because this code is tied to combustion venting safety. The furnace is shutting down to prevent operation when draft is not correct. While the error itself is protective, you should not ignore repeated lockouts, especially if you notice flue odor, unusual burner behavior, water buildup, or any signs of exhaust spillage.

What part usually fails with Error Code 31?

The pressure switch is often blamed first, but the most common issue is actually a blocked vent, clogged condensate drain, or moisture in the pressure tubing. The switch may be fine and simply responding to poor draft. The inducer motor is another common failure point if it cannot generate enough negative pressure.

How much does it cost to fix?

Costs vary with the actual cause. Clearing a vent or condensate line is usually on the low end, while replacing an inducer motor, pressure switch, or control board costs more. The key is accurate diagnosis first, because replacing the pressure switch without fixing a vent or drainage problem often does not solve Carrier Furnace Error Code 31.

Can a dirty filter cause Error Code 31?

Not usually as the direct cause, since Code 31 is primarily a pressure switch and venting fault. However, a severely dirty filter can contribute to overheating, excess condensate, and unstable furnace operation. It is worth replacing the filter during troubleshooting, but focus first on vent pipes, the inducer, condensate drain, and pressure switch hose.

Should I bypass the pressure switch to test the furnace?

No. Bypassing a pressure switch is unsafe and can allow the furnace to run without proper venting verification. That exposes you to combustion and carbon monoxide risks and can damage equipment. If you suspect the switch or PCB is faulty, the safe approach is proper meter testing and pressure verification by a qualified technician.