What each part does
The igniter lights the gas. On most furnaces of recent decades it is a hot-surface igniter: a fragile element that glows bright orange for a few seconds before the gas valve opens. Older designs use a spark, and the oldest use a standing pilot flame that burns all the time.
The flame sensor proves the burners actually lit. It is a thin metal rod that sits in the flame. A tiny current flows through the flame to it, and if the control board does not see that current within a few seconds of opening the gas valve, it shuts the gas off. That is a safety working correctly: gas flowing with no flame is the thing a furnace must never allow.
How each one fails
| What you see | Usual part |
|---|---|
| No orange glow through the inspection window, burners never light | Igniter, or the board not powering it |
| Glow, then gas, then no flame | Igniter not hot enough, or a gas supply problem |
| Burners light, run a few seconds, shut off, try again | Flame sensor |
| Works some days and not others, worse when it is cold | A cracked igniter or a fouled sensor, both intermittent at first |
Hot-surface igniters wear out and crack, and they are easily broken by a touch. Flame sensors do not usually break; they get a film on them from combustion and dust until the current is too weak to read.
What the repair involves
A flame sensor is usually cleaned or replaced, and its current measured afterwards with a meter so the repair is proven rather than hoped. An igniter is tested for resistance, and replaced if it has failed or cracked. Both are inexpensive parts compared with the rest of a furnace.
The thing to watch for is a sensor that keeps fouling quickly after cleaning. That can point at a combustion problem rather than a dirty rod: poor air supply, a burner out of adjustment, or, at this altitude, an appliance that was never set up for the elevation. The altitude guide covers that last one.
Leave both alone
An igniter element shatters if it is touched, and the flame sensor sits inside the burner compartment behind the gas valve. Neither is a homeowner repair. Note what you see through the inspection window, and whether a fault light is blinking, and pass that on.
A standing pilot
On an older furnace with a pilot light, the equivalent part is the thermocouple, which proves the pilot is lit. A pilot that lights and will not stay lit after you release the button is the classic thermocouple fault. If you are not sure whether you have a pilot, the label inside the lower panel usually says, and the not heating page walks through the symptoms.
No heat? Tell us what the furnace is doing.
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