Quick checks
- Thermostat: set to Heat, set above the current room temperature, with fresh batteries if it uses them.
- Filter: a clogged filter can cause the furnace to overheat and shut itself off on a safety limit. See filter guide.
- Power: furnaces have a light-switch-style power switch near the unit, plus a breaker. Make sure both are on.
- Panel door: the blower door has a safety switch, so the furnace won't run if the door isn't seated properly.
- Gas supply: if other gas appliances work, the supply is likely fine.
Common failures a technician finds
- Dirty flame sensor. The furnace lights, then shuts off after a few seconds. A common, inexpensive fix.
- Failed igniter. Modern furnaces use a hot surface igniter that eventually cracks.
- Pressure switch or inducer motor problems. Often triggered by a blocked vent or condensate line on high-efficiency furnaces.
- Blower motor or capacitor failure. The burners run but no air moves, or air moves weakly.
- Control board faults. Many furnaces flash a diagnostic code on an indicator light. Note the pattern for the technician.
When to stop
If you smell gas, see scorch marks, hear booming at ignition, or your CO detector sounds, turn the system off, get everyone out if needed, and call for help. These aren't DIY situations. For everything else, schedule heating repair with a licensed contractor.