Carrier Furnace Error Code 43 usually indicates a pressure switch or venting fault that prevents the control board from confirming proper inducer draft during ignition. The most likely fix is to inspect and clear the pressure switch tubing, vent pipe, and condensate drain, then verify the inducer motor is running correctly.

⚡ Quick Fix

1. Turn the furnace off at the disconnect or breaker, wait 15 to 30 minutes, then restore power and call for heat at the thermostat. This hard reset can clear a temporary control board lockout caused by a brief draft or ignition fault. If the code returns immediately, continue with airflow and pressure switch checks.
2. Inspect the pressure switch hose, vent pipe, intake pipe, and condensate drain for blockage, water, cracks, or loose connections. On Carrier condensing furnaces, a partially blocked drain trap or water in the pressure tubing is one of the most common real causes of Error Code 43. Correct any obstruction and restart the unit.
3. If the error remains, check the inducer motor, pressure switch continuity, wiring harness connections, and the furnace control board signal circuit with a multimeter. If the inducer does not establish proper negative pressure or the switch does not close reliably, stop and schedule professional service.

What This Error Means

On a Carrier Furnace, Error Code 43 generally points to a problem in the draft proving circuit. The furnace control board expects the inducer assembly to create the correct pressure difference, which closes the pressure switch and confirms safe venting before ignition begins. If that sequence fails, the PCB stops the heating cycle to prevent unsafe combustion or flue gas spillage.

Technically, this code is often triggered when the pressure switch stays open, opens unexpectedly during operation, or the control board does not see the proper signal from the switch circuit. Restrictions in the intake or exhaust, a weak inducer motor, water in the condensate system, damaged tubing, a stuck switch diaphragm, or a wiring harness problem can all cause the same fault. In some cases, the issue is intermittent and only appears during cold weather, high wind, or heavy condensate production.

Because Carrier Furnace Error Code 43 is tied to combustion safety, it should not be ignored or repeatedly reset without diagnosis. The furnace may start briefly and then shut down, or it may lock out completely. The goal is to determine whether the problem is airflow, condensate, electrical continuity, or a failed component in the pressure proving circuit.

Common Causes

Blocked vent or intake piping

Leaves, snow, insect nests, debris, or a collapsed section of PVC venting can reduce inducer draft enough to keep the pressure switch open. Even a partial restriction can alter negative pressure readings and trigger Error Code 43 during startup.

Clogged condensate drain or water in pressure tubing

High-efficiency Carrier furnaces produce condensate that must drain freely through the trap and hose assembly. If the trap is blocked or water backs up into the pressure switch tubing, the switch may not respond correctly and the control board will stop ignition.

Failed or weak inducer motor assembly

The inducer motor must create stable draft before the ignition sequence can continue. A worn motor, damaged wheel, weak capacitor on applicable assemblies, or excessive bearing drag can reduce airflow and prevent the switch from closing consistently.

Defective pressure switch

The pressure switch contains a diaphragm and electrical contacts that react to inducer vacuum. If the diaphragm is stiff, the contacts are pitted, or the switch calibration has drifted, it may fail even when venting is clear.

Loose wiring harness or control board fault

A corroded terminal, loose spade connector, broken wire, or failed relay/input on the furnace control board can mimic a draft proving problem. This is less common than blockage or condensate issues, but it becomes more likely after vibration, moisture exposure, or previous repair work.

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 visual inspection. Shut off power to the furnace at the breaker or service switch, wait 15 to 30 minutes, then remove the access panels and look for obvious issues such as loose hoses, disconnected wiring, standing water, or burned connectors on the control board. Restore power and call for heat so you can observe whether the inducer starts before the code returns.
2. Inspect the intake and exhaust venting outside and at the furnace cabinet. Remove any leaves, ice, snow, nests, or debris from the pipe terminations, and check that no section of the vent is sagging and holding water. If the furnace is in a high-wind area, look for signs that venting is being affected by weather, as this can create intermittent pressure switch faults.
3. Check the condensate system carefully on high-efficiency models. Remove and clean the condensate trap if accessible, flush drain lines, and confirm that water moves freely without backing up toward the pressure switch ports. If you find moisture inside the pressure switch tubing, dry or replace the tubing and correct the source of the water intrusion before restarting the furnace.
4. Examine the pressure switch hose and ports. Make sure the rubber or silicone tubing is not cracked, brittle, pinched, soft from heat, or clogged with debris, and verify it is firmly connected at both the inducer housing and the switch. Also inspect the pressure port on the inducer assembly because even a small blockage there can prevent the switch from sensing proper vacuum.
5. Test the inducer motor operation. When the thermostat calls for heat, the inducer should start promptly and run smoothly without grinding, hesitation, or abnormal vibration. If it hums, starts slowly, overheats, or fails to maintain speed, the motor or inducer assembly may be the root cause and should be professionally tested for voltage, amp draw, and mechanical condition.
6. Use a multimeter to check the pressure switch circuit only if you are comfortable working around electrical components. With power off, inspect continuity through the wiring harness and terminals; with the proper live test procedure, a technician would verify whether the switch closes when the inducer creates draft. Do not bypass the pressure switch for normal operation, because it is a critical combustion safety device.
7. Inspect the furnace control board and low-voltage connections. Look for scorched spots, moisture damage, loose plugs, cracked solder joints, or a blown fuse on the PCB. If the switch and inducer test good but the board does not read the signal correctly, the control board or wiring may be faulty.
8. Stop and call a professional if the venting is clear, the drain is open, the tubing is intact, and Error Code 43 still returns. A licensed HVAC technician can measure inducer draft with the correct instruments, verify pressure switch setpoints, test the heat exchanger and venting design, and rule out board-level failures safely.

Related Errors

• Error 31 — pressure switch did not close or draft was not proven during the ignition sequence.
• Error 33 — limit or flame rollout switch is open, often caused by overheating or restricted airflow.
• Error 34 — ignition proving fault or ignition lockout after repeated failed burner starts.
• Error 41 — blower motor fault indicating the indoor fan is not operating as commanded.
• Error 42 — inducer or vent pressure issue involving the draft system and pressure switch logic.
• Error 13 — limit circuit lockout after repeated open limit events requiring investigation of airflow or overheating.

FAQ

Can I reset Carrier Furnace Error Code 43 without a technician?

Yes, you can try a basic reset by shutting off power to the furnace for 15 to 30 minutes and then restoring power. If the code was caused by a temporary lockout, it may clear. If the fault returns, the furnace still has a real pressure switch, venting, inducer, or condensate problem that needs diagnosis.

Is Error Code 43 dangerous?

It can be, because this code is related to safe venting and combustion draft. The pressure switch circuit exists to confirm that flue gases can exit properly before and during burner operation. Do not keep forcing the furnace to run if the code keeps returning, especially if you notice exhaust odor, water leaks, or incomplete ignition.

What is the most common cause of Carrier Furnace Error Code 43?

The most common cause is restricted draft from a blocked vent, clogged condensate drain, or water-filled pressure switch tubing. These problems are especially common on condensing furnaces during cold weather. A dirty or failing inducer motor is another frequent cause when the furnace cannot create enough negative pressure to close the switch.

Can a dirty filter cause Error Code 43?

Usually not directly. A dirty air filter more often causes high-limit faults by restricting airflow across the heat exchanger, but it does not typically create a pressure switch draft error. Still, replacing a clogged filter is good maintenance, because poor airflow can compound other furnace problems and reduce overall system performance.

How much does it cost to fix Carrier Furnace Error Code 43?

Cost depends on the exact failure. A simple vent or condensate cleaning may be relatively inexpensive, while replacing an inducer motor, pressure switch, or control board costs more. Diagnosis matters, because replacing parts without confirming vent pressure, electrical continuity, and drain condition can waste money and leave the original fault unresolved.

Should I replace the pressure switch myself?

Only if you have confirmed that venting, condensate drainage, tubing, and inducer performance are all correct first. Pressure switches often get replaced when the real problem is blockage, water, or a weak inducer. If you are not comfortable testing live circuits and combustion-related components, professional service is the safer choice.