Furnace Not Turning On or Won't Ignite: 6 Common Causes
You hear the furnace start but the burners never light, or the whole system stays silent.
Likely causes
| Cause | Likelihood | Typical cost |
|---|---|---|
| Dirty flame sensor | 35% | $140 – $240 |
| Faulty hot-surface ignitor | 20% | $220 – $420 |
| Gas valve issue | 10% | $380 – $780 |
| Thermostat wiring | 10% | $120 – $260 |
| Blocked flue / pressure switch | 15% | $180 – $520 |
| Control board failure | 10% | $450 – $900 |
Safe DIY checks (do these first)
- 1
Verify the gas is on
Check that the gas shutoff valve at the furnace is parallel to the pipe (open) and that other gas appliances (stove, water heater) work.
- 2
Check the thermostat
Mode = HEAT, setpoint at least 3°F above room temp, fresh batteries.
- 3
Look at the furnace status light
Most furnaces have a blinking LED behind a small window. Count the flashes and check the code chart inside the door panel.
- 4
Try a reset
Flip the furnace power switch OFF for 60 seconds, then back ON. If it lights, monitor for a repeat.
- 5
Inspect the filter
A very dirty filter can cause overheating that trips the high-limit safety and prevents ignition.
When to stop and call a pro
- • Gas smell — leave the house first and call from outside
- • Furnace tries to ignite 3 times then locks out
- • Popping or delayed ignition sounds
- • Yellow flames instead of blue
- • Any repair involving gas piping or the heat exchanger
Typical repair cost
$180 – $950. Flame sensor cleaning is the most common and least expensive fix.
Frequently asked questions
Why is my furnace not turning on at all?
No response at all usually means power or the control circuit: a tripped furnace breaker, an open blower-door safety switch, a blown 3-amp fuse on the control board, or a dead thermostat. If the blower runs but the burners never light, the problem is ignition instead — flame sensor, igniter, or pressure switch.
How do you light the pilot on a furnace?
Only older standing-pilot furnaces have one. Turn the gas control knob to OFF, wait five minutes for gas to clear, set it to PILOT, hold the reset button down while lighting the pilot with a long lighter, keep holding 30–60 seconds after it lights, then turn the knob to ON. If the pilot will not stay lit, the thermocouple has failed — that is a $145–$265 repair, not a DIY part swap. Never attempt this if you smell gas.
How much does furnace pilot light repair cost?
Thermocouple replacement runs $145–$265 installed. A pilot assembly or gas valve with a failed pilot circuit runs $265–$795. On modern furnaces there is no pilot at all — the equivalent failures are the hot-surface igniter ($245–$395) and the flame sensor ($185–$265).
Is a furnace that won't ignite dangerous?
The furnace itself is designed to fail safe — but never bypass safety switches or leave a gas smell unaddressed. Call (661) 310-3340 or 911 if you smell gas.
How long does furnace ignition repair take?
Most ignition repairs (flame sensor, ignitor, pressure switch) finish in 45–90 minutes on-site including diagnosis.
Why does my furnace only fail on cold mornings?
Marginal flame sensors and weak ignitors fail first on cold starts because more voltage/heat is required. It's an early sign the part is at end of life.
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