York / Coleman / Luxaire · Furnace error code

4 red flashesHigh limit open (some boards: 24 V fuse open)

Cross-checked High-voltage / caution

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

Related diagnostic guides

Guide · Furnace

Furnace runs no heat (blower only or nothing)

Start with the board LED code, then work the call path: thermostat, 24 V, safeties, inducer, ignition, flame proving. Most no-heat calls end at a filter, a flame sensor, a pressure switch issue or gas being off.

5 steps ~45 minmoderate

Guide · Furnace

Limit trips / overheating (airflow)

The limit protects the heat exchanger from overheating. The fix is almost always airflow: filter, blower, ducts. Rollout switch trips are different and must be referred.

4 steps ~45 minmoderate

Guide · Furnace

Furnace blowing cold air on a heat call

The common causes are a fan set to ON, a dirty filter tripping the high limit so the burners stop while the blower runs, a full condensate drain on a high-efficiency furnace, or an ignition lockout. Gas valve and burner work is blocked; stop at diagnosis and refer.

4 steps ~45 minmoderate