Gas Furnace: No Heat
The furnace is calling for heat but the burners never light, or they light and then shut down.
Safety
Thermostat Call: The thermostat closes R to W, sending a 24V call for heat.
Symptom
Symptom
The thermostat is calling for heat and the furnace either does nothing, runs the inducer and stops, or lights briefly and then shuts down.
Work this fault in the order the furnace itself works. Each stage of the sequence of operation has to prove before the next one is permitted, so the stage that stops is the stage that failed.
What you observe
- Nothing happens at all - no inducer, no board activity.
- The inducer runs continuously but nothing else happens.
- The ignitor glows, the burners light, and flame drops out after a few seconds.
- The furnace runs a short cycle then shuts down on a limit.
Possible causes
- No 24 volt control power - failed transformer, tripped door switch, or blown low voltage fuse.
- Blocked vent, blocked condensate trap, or a failed pressure switch preventing draft proof.
- Weak or cracked hot surface ignitor.
- Dirty or failed flame sensor producing a low flame signal.
- Gas supply problem - closed valve, failed gas valve, or low supply pressure.
- Restricted airflow opening a limit switch.
What to check
Read the control board fault code before disturbing anything
The board already recorded which stage failed - reading it first saves most of the diagnostic time.
Measure 24 volts between R and C at the board during a call
No control voltage points to the transformer, fuse, or a safety in the control circuit.
Measure pressure switch closure with the inducer running
An open switch with the inducer running points to the vent, the trap, the hose, or the inducer itself - not to the switch.
Measure flame signal in microamps during the trial for ignition
A reading below the manufacturer minimum explains a flame that lights and drops out.
Measure temperature rise across the heat exchanger
A rise above the data plate range means airflow is the underlying problem, not the limit switch that tripped.
Next step
Correct the condition the failing stage revealed, then run the furnace through a full cycle and confirm every stage proves normally. Verify temperature rise is within the data plate range before leaving.
Safety note
Never bypass a safety
Local Code Notice
Symptoms
Furnace won't start
Diagnosed here
Furnace won't ignite
Diagnosed here
Furnace blows cold air
Diagnosed here
Step-by-step: Gas furnace: no heat
Work through the checks in order rather than guessing.
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