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Blowing cold air
What is going on
The blower runs but the air coming out of the registers is cold.
Likely causes
- Thermostat fan set to On instead of Auto
- Burner not lighting or locking out
- Limit or pressure switch open while the blower keeps running
- Gas supply problem
Check in this order
- Set the thermostat fan to Auto
- Check the control board LED for a lockout or limit code
- Confirm gas supply and that the burners light
- Check the condensate trap and pressure switch tubing
- Verify temperature rise once the burner runs
Applies to: Gas furnaces
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Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.