Digital thermostat mounted on a concrete wall

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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.

Cross-checked ~45 min moderateBrands: All

Before you start

Gas smell or CO alarm: leave and call the gas utility or 911.
Gas valve, burner and gas pressure work is blocked: refer to a licensed HVAC technician.
Turn off power before opening the burner compartment.

Diagnostic steps

Unlock all 4 steps with expected values

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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.

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