Furnace Won't Turn On? Causes and Fixes
A furnace that won't start usually comes down to power, thermostat, filter or ignition — here's how to diagnose it safely.
When your furnace won't turn on, the cause is often something simple you can check before calling a technician. Work through these steps in order, and stop at anything involving gas or wiring you're not comfortable with.
Start with power and the thermostat
Confirm the furnace switch (it looks like a light switch near the unit) is on, the breaker hasn't tripped, and the thermostat is set to HEAT with the temperature above room temperature. Replace thermostat batteries if the screen is blank.
Check the filter and safety switches
A clogged filter can trip the limit switch and shut the furnace down. A loose furnace door also disables the safety door switch. Swap a dirty filter and make sure the panel is seated firmly.
Key takeaways
- Verify the furnace power switch and breaker first
- Set the thermostat to HEAT, a few degrees above room temp
- Replace a dirty filter — it's the #1 nuisance cause
- Make sure the front access panel is fully closed
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