Symptoms
- Furnace is completely silent
- No fan, no ignition, nothing happens
- Thermostat display may be blank or unresponsive
- Breaker or switch may be off
Likely causes
- CommonThermostat set wrong or dead batteriesA dead thermostat can't call for heat even if the furnace works fine.
- CommonTripped breaker or blown fuseFurnaces run on a dedicated circuit that can trip like any other.
- CommonFurnace power switch or door switch offFurnaces have a light-switch-style shutoff nearby and a safety switch on the door panel.
- PossibleClogged air filter tripping a safety limitA very dirty filter can cause overheating that shuts the unit down.
- Less commonFailed control board or transformerElectrical components can fail with age and stop the whole system.
What to do
- Check the thermostat is set to Heat with a temperature above room temperature, and replace batteries if the screen is dim or blank.
- Find the furnace's electrical switch (looks like a light switch, often near the unit) and confirm it's on.
- Check the front panel is fully seated and latched; most furnaces won't run if the door safety switch isn't engaged.
- Go to the breaker panel and check for a tripped breaker; reset it once by fully switching off then on.
- Check the filter; if it's gray and clogged, replace it and try again.
- If it's a gas furnace, confirm the gas shutoff valve on the line is open.
- If none of this works, avoid repeatedly resetting the breaker and call an HVAC technician.
Tools and parts
- Flashlight
- Replacement furnace filter
Standard furnace air filterSearch: “furnace air filter size replacement”
When to call a pro
If power, thermostat, filter, and breaker all check out and it still won't start, call an HVAC technician to test the control board and transformer.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.