Ignitor vs flame sensor: which one failed?
If the burners never light, suspect the ignitor. If they light and then shut off within a few seconds, suspect the flame sensor.
The ignitor heats until it glows (or sparks) so the gas can ignite. The flame sensor is a thin metal rod that proves a flame is present; without that proof the gas valve closes for safety.
Side-by-side
| Hot surface ignitor | Flame sensor | |
|---|---|---|
| Job | Lights the gas | Confirms the flame |
| Failure sign | No orange glow, no ignition | Lights, then goes out in seconds |
| Typical cause | Cracks with age and heat cycles | Carbon and oxide coating |
| Usual fix | Replace | Clean, or replace if pitted |
| Handling note | Never touch silicon nitride/carbide elements with bare fingers | Light abrasive pad only, never sandpaper |
Why ignitors fail in early winter
Silicon carbide ignitors are brittle and develop hairline cracks after a few years of heating cycles. They often fail on the first cold morning after sitting idle all summer.
Many technicians keep universal ignitors on the truck, so this is often a one-visit repair.
What happens during the repair
- Power off and gas off at the appliance valve.
- Remove and inspect the ignitor or sensor.
- Measure ignitor resistance or flame microamps to confirm the diagnosis.
- Install the replacement without touching the element.
- Restore gas and power and watch several full ignition cycles.
Signs it's not just the ignitor
- New ignitor also fails within weeks (check voltage and the board)
- Burners light on only one side (crossover or gas-pressure issue)
- Yellow, lifting or rolling flames (combustion problem)
- Flame sensor keeps coating quickly (dirty air or poor combustion)
Older furnaces with a standing pilot
Furnaces from before the mid-1990s may use a standing pilot light and thermocouple. If the pilot won't stay lit, the thermocouple is the usual culprit. Relighting instructions are on the furnace label. If you smell gas, don't relight it; leave and call the utility.
Ignition FAQs
How long do furnace ignitors last?
Often 3–7 years for silicon carbide and longer for silicon nitride, depending on how many cycles the furnace runs.
Can I clean a flame sensor myself?
Many homeowners do, with power and gas off and a light abrasive pad. If you aren't comfortable working inside the furnace, have a technician do it during a tune-up.
Next steps
Ready to get it fixed? Call (888) 555-0141 to reach a licensed provider in your area. Related guides:
Busy winter markets: furnace repair in Minnesota · furnace repair in Illinois · furnace repair in Michigan · furnace repair in Ohio · furnace repair in Pennsylvania · furnace repair in New York · all states.