Furnace: No heat
No heat on a furnace: Thermostat is calling for heat but the furnace does not start or does not light.
Applies to: Gas furnaces
Common causes of no heat
- Thermostat setting, batteries or wiring
- Power switch, breaker or blown low-voltage fuse
- Gas supply off or low pressure
- Dirty flame sensor or failed igniter
- Blocked vent, intake or condensate trap tripping the pressure switch
- Limit or rollout switch open
How to fix no heat: step by step
- Confirm the thermostat is in Heat and set above room temperature; check batteries
- Check the furnace power switch, breaker and the 3 or 5 A fuse on the control board
- Read the LED flash code on the control board and look it up under Error Codes
- Confirm the gas valve is on and check inlet gas pressure
- Watch the ignition sequence: inducer, pressure switch, igniter glow, flame, flame proving
- Clean the flame sensor and check flame microamps if the burner lights then shuts off
- Clear the condensate trap and check vent and intake terminations for blockage
- If a rollout switch has tripped, inspect the heat exchanger and venting before resetting
If the unit shows an error code, look it up: AirQuest Furnace codes Carrier Furnace codes Daikin Furnace codes Goodman Furnace codes ICP Furnace codes KeepRite Furnace codes Lennox Furnace codes Midea Furnace codes Rheem Furnace codes Trane Furnace codes York Furnace codes
Source: General HVAC service practice — verify per model. Written in our own words as a summary; always check the original manual.
General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.