Error code reference

Carrier Furnace Error Code 21

Gas heating lockout; control will NOT auto-reset.

First step from the service manual

Turn off power and wait 5 minutes to retry. Check for mis-wired gas valve, defective control (valve relay), or short to gas valve wire.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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Gas heating lockout

From Service Manual 102617, p.29:

1.Turn off power and wait 5 minutes to retry. Check for mis-wired gas valve, defective control (valve relay), or short to gas valve wire.

2.Measure door lock assembly resistance across the PTC terminals. Expected: ~200 ohms cold, rising sharply once energized.

3.Inspect strike and latch alignment. A warped door or misaligned strike prevents the switch stack from closing in the correct sequence.

Part W10810403

Door lock assembly

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