Here's the classic flame sensor symptom: the furnace fires up, burns for two or three seconds, then shuts off — and repeats before locking out. The flame sensor proves to the control board that the burner is actually lit. When it's coated with oxide buildup or the rod is cracked, it can't pass enough microamp current and the board shuts off the gas. Try cleaning it with steel wool first — but if cleaning doesn't fix it, the sensor needs replacing. Backed by a 1-year warranty and priced up to 50% less than dealer pricing.
