Carrier furnace error codes

Official: check for 115VAC at L1/L2 and 24VAC at SEC-1/SEC-2. No light means the board isn't powered.

Official: line-voltage polarity is reversed — and the control blocks heating until it's fixed. (If the furnace is twinned, see the twinning-kit instructions.)

1 short + 1 long flash on recall = nothing stored. Codes self-erase after 72 hours (and on RED-LED boards, whenever power is interrupted).

The furnace was powered up in the middle of a call for heat (R-W closed), so the blower runs 90 seconds first. Informational, not a fault.

A limit, draft safeguard, flame rollout, or blocked-vent switch stayed open longer than 3 minutes → lockout. Auto-resets after 3 hours. Root causes = see code 33.

The system failed to ignite and prove flame in 4 attempts. Auto-resets after 3 hours. Root causes = see code 34.

Official: mis-wired gas valve, a stuck-closed gas-valve relay on the control, or a shorted gas-valve wire. This one does NOT auto-reset.

Flame is proved while the gas valve is de-energized; the inducer runs until the fault clears. Official causes: a leaky or stuck-open gas valve.

Official: obstructed pressure tubing or a pressure switch stuck closed.

A short in the 24VAC wiring blew the board's automotive-type 3-amp fuse. Official: replace with a 3-amp fuse ONLY.

If the switch stays open over 5 minutes, the inducer rests 15 minutes and retries. Official checklist: excessive wind, low inlet gas pressure (if LGPS used), disconnected or obstructed pressure tubing, defective switch.

A limit, draft safeguard, flame rollout, or blocked-vent switch is open. The blower runs 4 minutes or until it closes; open >3 minutes escalates to lockout code 13. Rollout and BVSS switches need a manual reset.

No flame proved during the trial for ignition; the control retries 3 more times before lockout 14. Official checklist: flame sensor must not be grounded, oxide buildup (clean with fine steel wool), proper microamps (0.5 min, 4.0–6.0 nominal), gas valve on, igniter health, inlet gas pressure.

Auto-resets after 1 hour. Official causes: gas-valve relay stuck open, flame-sense circuit failure, or a software check error. Reset power to clear; replace the control if it repeats.

On the two-stage variable-speed control (58MVP, 58MVB, 58UVB), the blower failed to reach 250 RPM, or failed to talk to the control within 30 seconds of being switched on, in two heating cycles in a row. Official: the lockout auto-resets after 3 hours, and the root causes are the same as code 41.

Official: if code 25 flashes only 4 times at power-up, the model plug PL4 is missing and the control is running on the model stored in memory, and the furnace operates properly. If it flashes continuously: PL4 missing with no valid model in memory (typical after a replacement board), a thermostat call while setup switch SW1-1 or SW1-6 is ON, both switches ON together, or two different furnace models twinned.

On the two-stage variable-speed control, the low-heat pressure switch did not close within 45 seconds of a call for low heat, or it opened during low heat (a low gas pressure switch opening reads the same). The control shuts down, waits 2 minutes and retries. If the switch opens within 5 minutes after ignition, the next cycle runs in high heat only. Official checklist: vent sizing, plugged condensate drain, low inlet gas pressure (if LGPS used), excessive wind, restricted combustion air, pressure switch wiring, failed or out-of-calibration switches, water or sag in the vent piping, disconnected or obstructed pressure tubing.

On the two-stage variable-speed control, the blower failed to reach 250 RPM or failed to talk to the control within 30 seconds of being switched on (10 seconds during steady-state running). Official first checks, power off: a blower wheel rubbing the housing, a loose wheel on the motor shaft, and the wiring from the control to the motor. After that the flowchart is a voltmeter test at each plug that ends at the blower control module, the motor, or the furnace control.

On the two-stage variable-speed control, the inducer did not start within 20 seconds of a call for heat, its RPM is outside the valid range, or the RPM signal was lost for 5 seconds while running. Official checklist: proper vent sizing, restricted combustion air supply, failed inducer motor, improper motor wiring.

On the two-stage variable-speed control, a call for high heat closed the high-heat pressure switch but the low-heat switch stayed open. The control shuts down, stops the inducer, keeps the gas off, and retries after 2 minutes. Official checklist: low inlet gas pressure (if LGPS used), plugged condensate drain, pressure switch wiring, water or sag in the vent piping, a stuck-open low-heat switch, disconnected or obstructed pressure tubing.

On a Carrier 59MN7A, code 35 means the modulating gas valve did not answer a command from the furnace control, or power to the valve electronics was interrupted. The control shuts the furnace down, waits 2 minutes, and restarts the heating cycle.