Furnace won't ignite

When a furnace fails to light, first check whether it has a heat call and power. Startup can stop during draft proving, ignition or flame proving. Check thermostat settings and record any code. A technician handles ignition, gas and flame-sensing tests.

Startup runs in steps. The control needs a heat call and power, then confirms draft through the pressure switch, tries ignition and checks that flame is present. A stall at any step can look like the same problem. A furnace can also light and then shut off because flame was not proved, so no ignition does not automatically mean a bad igniter.

This page covers a furnace that is calling for heat but not lighting or not staying lit at startup. Rooms that are simply cold, or air that blows cold, have separate pages.

Safe checks before calling

Thermostat mode and setpoint
Confirm the thermostat is on HEAT, the setpoint is above room temperature, and no schedule or hold is overriding it. If it runs on batteries and its manual allows, replace them. Do not keep raising the setpoint to force another start.
Service switch and breaker
Look at the furnace service switch and the labeled breaker and note their positions for the technician. A lit thermostat screen does not prove the furnace has power. Do not reset a tripped breaker or open the electrical panel or furnace.
Filter and room grilles
With the furnace off, check an easily reached external filter. Replace a dirty disposable filter with the size your owner's manual specifies, put covers back, and never run without a filter. Clear furniture or curtains from supply and return grilles.
What you heard and saw
Note whether the heat call produced silence, a startup sound or click that then stopped, or brief warmth that faded. Record the full model, serial and any LED pattern through the viewing window, without removing panels or retrying the startup.

Where startup can stop

No heat call or no power

If the thermostat is not calling or the furnace has no power, nothing in the sequence starts. A blank control light points this way, but the cause can sit anywhere from the switch to the fuse.

Draft not proved

The control waits for the pressure switch to confirm the inducer has established draft. Vent restrictions, tubing, the inducer, wiring or the switch can hold up this step, and ignition will not be attempted until it passes.

Ignition without proved flame

Burners can light briefly and still shut down if the control does not sense flame. On Heil G80CTL Series A, six amber flashes mark a failed trial and 6+1 marks lockout after four trials.

Ignition and flame-sensing faults

Gas supply, the gas valve, the igniter, grounding, flame sensing and flame carryover are all possible directions for a technician. A click with no heat, or a lockout code, does not by itself identify which one.

When to call a technician

Call for service if basic checks do not restore heat, if you see a lockout code, or after repeated failed starts. Stop using the furnace for loud ignition bangs, soot or a damaged vent pipe. A technician checks power, draft proving, ignition and flame proving against the model's sequence and handles all gas and ignition parts.

Repair quotes

Common questions

My furnace clicks but never heats. What does that mean?

A click or startup sound only tells you roughly where the sequence stopped. It cannot show whether the gas valve, igniter or flame sensor is involved. Note what you heard and any code, and give that to the technician.

Should I keep resetting the furnace after it fails to light?

No. A lockout means a safety or flame-proving condition was not met. Repeated power cycling, resets or forced restarts do not fix that condition. Record the code and call for service instead.

Is it just a dirty flame sensor?

Possibly, but a code cannot confirm that. Flame proving also depends on gas delivery, grounding and how the flame reaches the sensor. Cleaning or removing the sensor is technician work, not an owner task.

Sources

Reviewed 2026-10-07 · How we verify codes