Furnace won't ignite: ignitor, gas, or flame sensor?
A furnace that will not ignite most often has a worn hot surface ignitor, a dirty flame sensor, or no gas reaching the burners. Watch one start-up: if the ignitor never glows, suspect the ignitor; if it glows and no flame appears, suspect gas supply; if flame appears and dies within seconds, suspect the flame sensor. The repair itself is a technician's job.
Safety first. If you smell gas, do not try to restart the furnace. Leave the house and call your gas utility or 911 from outside. Do not reset a furnace that fails to light more than once, because unburned gas can collect in the burner area.
Check these first
0 of 6 checkedStill not working after these? That is the point to bring in a licensed technician.
What it usually means
Modern furnaces light the burners with a hot surface ignitor, a small ceramic element that glows orange, or with a spark. The control board allows a short window for the flame to appear and be confirmed. If that does not happen, it closes the gas valve, waits and tries again. After a set number of failed tries it locks out for a period of time.
What you see and hear through the sight window during one attempt narrows the cause a great deal. Watch with the furnace door closed. Removing the door trips a safety switch, and the burner compartment is no place for hands or tools while the furnace is powered.
Read the start-up
| What happens | Most likely cause |
|---|---|
| Inducer fan runs, the ignitor never glows | Failed ignitor, or a pressure switch that has not closed |
| Ignitor glows, no flame | No gas: closed valve, supply problem or a failed gas valve |
| Clicking, no flame | Spark ignition trying with no gas, or a spark that is not reaching the burner |
| Flame lights, then goes out within seconds | Dirty or failed flame sensor |
| Nothing at all | Power, thermostat or lockout |
Likely causes, most common first
- Worn ignitor. Hot surface ignitors are wear parts. They crack or weaken with age and stop reaching the temperature needed to light gas. A crack is often too fine to see.
- Dirty flame sensor. The burners light and shut off right away. Details are in bad flame sensor.
- Pressure switch open. A blocked vent, a blocked condensate drain or a weak inducer motor stops the sequence before ignition.
- Gas supply. The shutoff valve is closed, the propane tank is low, or utility service is interrupted.
- Dirty burners or a failed gas valve. Less common, and both need a technician.
- Control board. The board fails to send power to the ignitor or the gas valve.
From the outside you can narrow the fault to one or two of these. You cannot confirm it. A cracked ignitor and a board that is not powering the ignitor look identical through the sight window, and only a meter reading tells them apart. That is why replacing parts on a guess often costs more than a diagnosis.
What a technician will do
The technician measures the ignitor's resistance and the voltage reaching it, checks the pressure switch and vent, confirms gas pressure at the valve and measures the flame sensor signal. Replacing an ignitor is usually quick once the diagnosis is confirmed. Ignitors are fragile and matched to the furnace model, so the part and its handling matter. The ignitor replacement cost guide covers what goes into the price.
When it points to replacement
An ignition fault is almost always a repair. Consider replacement only if the technician also finds a damaged heat exchanger, heavy rust in the burner compartment, or a failed control board on an old furnace that has needed other work. See the furnace repair guide for how to weigh that.
Common questions
What are bad furnace ignitor symptoms?
The inducer fan starts, then nothing glows and no flame appears, and the furnace retries a few times before locking out. Some weak ignitors glow dimly and fail to light the gas. Intermittent starts that get worse over days or weeks are also typical of an ignitor near the end of its life.
Why is my furnace clicking but not igniting?
Clicking is usually a spark ignitor firing or the gas valve opening. If no flame follows, gas may not be reaching the burner, or the spark is weak or misplaced. Do not keep resetting it. Turn the furnace off and have a technician check the gas supply and ignition parts.
Can I replace a furnace ignitor myself?
This site does not recommend it. The ignitor sits in the burner compartment next to gas components, it runs on line voltage in many furnaces, and the element breaks if it is touched or bumped. A wrong part can also damage the control board. A technician can confirm the ignitor is the real fault first.
How long does a furnace ignitor last?
It varies with the ignitor material, how often the furnace cycles and how clean the burner area stays. Treat it as a wear part that will need replacing at some point in the furnace's life. A furnace that short cycles uses up ignitor life faster because every start is another heat-up.
Related guides
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