Igniters and Flame Sensors: The Fifteen-Minute Furnace Calls

Two small parts cause a disproportionate share of winter no-heat calls, and both live on the burner assembly. Knowing which is which turns a mystery into a fifteen-minute ticket.

The hot-surface igniter is a consumable — a brittle ceramic element that glows to light the burner and eventually cracks from thermal cycling. Symptoms: inducer runs, gas valve wants to open, no glow, no light-off. Check resistance against spec (typically 40–90 ohms for silicon carbide types); infinite means broken. Handle the new one by the base — skin oil shortens its life — and never touch the element.

The flame sensor doesn't fail so much as get dirty. It proves the burner is lit by conducting a tiny current through the flame; a few seasons of oxide coating and the signal drops below threshold, so the board lights the burner and then shuts it down seconds later. That short-cycle pattern — light, run briefly, drop out, retry — is the sensor's signature. Clean it with fine abrasive, re-seat, done. Replace it if cleaning stops holding.

Both belong on every truck in stock quantity. They're cheap at wholesale, and the alternative is a return trip through January traffic for a part that weighs less than the invoice.