Furnace Won't Start
When your furnace won't start, start with the basics: check that the thermostat is set to heat and above room temperature, verify the furnace switch is on, and check for tripped breakers. About 40% of "no heat" calls are resolved by these simple checks.
- 1Check thermostat settings - Set to "heat" mode with temp at least 3 degrees above current room temperature. Replace batteries if digital.
- 2Check the furnace power switch - There's usually a light switch on or near the furnace. Make sure it's on.
- 3Check circuit breakers - Find the furnace breaker and reset if tripped. Also check the blower motor breaker.
- 4Check the air filter - A severely clogged filter can trigger a safety shutdown. Replace if dirty.
- 5Check gas supply - For gas furnaces, verify the gas valve is open. Check if other gas appliances work.
- 6Look for error codes - Many furnaces have a blinking LED that indicates problems. Note the pattern.
- Thermostat issues
Dead batteries, wrong settings, or a faulty thermostat can prevent the furnace from receiving the signal to start.
- No power to furnace
Tripped breaker, blown fuse, or the furnace switch was accidentally turned off.
- Ignition failure
Pilot light is out (older furnaces) or the electronic igniter has failed (modern furnaces).
- Dirty flame sensor
A common issue where the furnace starts briefly then shuts off because it can't detect the flame.
- Safety switch triggered
Clogged filters, overheating, or a stuck door switch can trigger safety shutdowns.
- Gas valve or supply issue
The gas valve may be closed, or there's an issue with gas supply to your home.
- You smell gas - leave immediately and call your gas company
- The furnace starts then shuts off after a few seconds
- Error codes are flashing on the furnace control board
- You've checked all the basics and it still won't start
- The furnace is making unusual sounds (banging, screeching, rumbling)
- You see soot or black marks around the furnace