Component
Flame sensor
The flame sensor proves the burner is actually burning before the gas valve is allowed to stay open. It is a small rod in the flame that conducts a tiny current; when it fouls or fails, the furnace lights and then shuts off seconds later, over and over.
Short answer
The classic flame-sensor symptom is a furnace that lights, runs a few seconds, shuts off, and retries until lockout: the control opened the gas valve but did not see flame proven. A fouled sensor (coated with residue) is the most common cause and is cleaned or replaced by a technician. Flame-signal measurement, not guesswork, confirms it.
How flame sensing works
The sensor rod sits in the burner flame. Flame rectification passes a tiny DC current through the flame to the burner, which the control reads as proof of flame. The current is small, so a dirty sensor, a poor ground, or moisture on the path can drop the signal below the control's threshold.
The failure pattern
A marginal sensor lets the furnace light and then lose the signal, producing repeated short runs and flame-loss codes. Fouling builds up over a season, which is why the fault often appears in early winter after a summer of idle, and why annual maintenance includes cleaning the sensor.
Diagnosis
A technician measures the flame signal in microamps while the furnace runs, cleans the sensor, and checks the burner ground and wiring. If the signal is restored, the sensor stays; if the rod is cracked or worn, it is replaced.
Related
Questions people ask
Can I clean the flame sensor myself?
The sensor sits inside the burner compartment of a gas appliance; accessing it means opening the burner area, which is a technician's job. Cleaning with the proper abrasive is part of maintenance.
Why does the furnace work again after I reset it?
Resetting gives the control another trial, and a marginal sensor sometimes proves flame on a later attempt. The underlying fouling is still there and the fault returns.
Is flame loss dangerous?
The control is shutting the gas off because it cannot prove flame, which is the safe behaviour. The danger would be gas flowing without proof of flame, which is exactly what this safety prevents.
Need this looked at?
Call BC Wide with the brand, model number and anything on the display. Or
Request service online with this page attached.