Furnace Ignition Parts
Igniters, flame sensors, control boards and pressure switches — the parts behind "it clicks but won't light", what actually fails, and how to tell them apart before you buy anything.
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Which ignition part is actually failing?
Diagnose before you buy, because these three parts fail with three distinct symptoms and the wrong one costs you an afternoon. If the igniter never glows, it is the igniter or the board feeding it. If it glows and the burners never light, look at the gas valve or the pressure switch. If the burners light and die about five seconds later, that is the flame sensor, every time — the board proved flame, failed to see it, and shut the gas off.
The flame sensor is usually not broken. It is a steel rod that fails by getting dirty: a white oxide film builds up and blocks the four-to-ten microamp signal the board is listening for. Ten minutes with a Scotch-Brite pad fixes it for free, and the trade sells a great many sensors to people who needed an abrasive pad. Clean it before you order one.
Silicon nitride outlives silicon carbide by two to three times, and it is worth the few dollars. Carbide is effectively glass — it cracks if it is knocked, and it degrades a little on every heat cycle. Nitride survives handling and warms up faster. Universal nitride kits replace most carbide igniters; the element is the upgrade, and the mount and the plug are what have to match, which is why the kits ship six brackets.
A furnace that eats igniters has a second problem. An element rated for years should not fail annually — if yours does, look at what is making it run more often than designed. A dirty filter that trips the limit and restarts the cycle is running the igniter twice as often as its design life assumed.