Furnace troubleshooting
Furnace Won't Turn On? Troubleshooting No Heat
A furnace with no heat often comes down to a few quick checks before it's a real fault. Start here.
Common causes
Thermostat set wrong or dead batteries
You can checkWrong mode, a low setpoint, or dead thermostat batteries are a surprisingly common cause.
Clogged air filter
You can checkA filthy filter chokes airflow and can trip the furnace into a safety shutdown.
Failed igniter or flame sensor
Pro repairA cracked hot-surface igniter or dirty flame sensor stops the burners from firing or staying lit.
Tripped safety / control board
Pro repairA blocked flue, bad limit switch, or failing control board can lock the furnace out.
What you can check yourself
- Set the thermostat to HEAT, raise it 5°, and replace its batteries.
- Check the furnace breaker and the power switch on the unit (it looks like a light switch).
- Replace a dirty filter.
- Make sure all supply/return vents are open and unblocked.
When to call a pro
If the basics check out and it still won't fire — or it lights then shuts off — it's usually the igniter, flame sensor, or control board. We diagnose and repair same-day.
Most furnace repairs run $150–$800 — see our repair cost guide.
Other furnace problems
FAQ
Furnace won't turn on / no heat — questions
A furnace with no heat often comes down to a few quick checks before it's a real fault. Start here. The most common causes are thermostat set wrong or dead batteries, clogged air filter, failed igniter or flame sensor.
You can safely check a few things first: Set the thermostat to HEAT, raise it 5°, and replace its batteries. Check the furnace breaker and the power switch on the unit (it looks like a light switch). Replace a dirty filter. If the basics check out and it still won't fire — or it lights then shuts off — it's usually the igniter, flame sensor, or control board. We diagnose and repair same-day.
Most furnace repairs run $150–$800 in the San Francisco Bay Area, parts and labor — you get a flat-rate quote before any work begins. See our repair cost guide for details.
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