When a furnace will not turn on, the system is usually protecting itself. Modern furnaces shut down when they detect unsafe ignition, venting, airflow, or flame conditions.
A few homeowner checks are worth doing first. After that, forcing repeated restarts can make the problem harder to diagnose.
Check these first
What you can safely check before calling
Check thermostat heat mode
Set the thermostat to Heat and raise the setpoint. Replace batteries if the thermostat screen is blank or dim.
Check the furnace switch and breaker
Many furnaces have a light-switch-style shutoff nearby. Confirm it is on, then check the breaker.
Replace the filter
A packed filter can trip safety limits. If the furnace overheats, it may start and shut down quickly.
Look for an error light
Many furnaces blink an error code through the front panel. Take a photo before turning power off.
Likely repair causes
What usually needs a technician
Failed ignitor
The furnace starts its sequence but never lights. Ignitors are common wear parts.
Dirty flame sensor
The burners light briefly, then shut off because the system cannot prove flame safely.
Pressure switch or venting issue
Blocked intake, blocked exhaust, condensate trouble, or inducer problems can stop ignition.
Control board or gas valve
If the ignition sequence is broken or gas is not opening safely, the furnace needs diagnosis.
Call if you see this
When to stop troubleshooting and schedule service
- You smell gas or suspect a gas leak.
- The furnace lights then shuts off repeatedly.
- The breaker trips again after reset.
- The error code returns after a filter change.
Next step
