Furnace part
Flame sensor
The flame sensor is a small metal rod that proves the burner is actually burning before the gas valve is allowed to stay open. It conducts a tiny rectified current through the flame; when it fouls or fails, the furnace lights and then shuts off seconds later, repeatedly.
Flame sensing uses flame rectification: the rod and burner pass a small DC current through the flame, which the control reads as proof of combustion. The signal is tiny, so a coated sensor, a poor ground or moisture on the path can drop it below the control's threshold.
Fouling builds over a heating season, which is why the classic symptom appears in early winter, and why annual maintenance includes cleaning the sensor with the proper abrasive.
Safety
The sensor sits inside the burner compartment. Cleaning or replacement is technician work; the control shuts the gas off when it cannot prove flame, and that protection is never bypassed.
Signs this part is involved
- Furnace lights, runs a few seconds, shuts off, and retries until lockout
- Flame-loss or ignition codes in the fault history
- Fault appears in early winter after a summer of idle operation
Related fault codes
- Carrier 59TP6C status code 14: flame-loss lockouts during a heat request
- Carrier 59TP6C status code 34: flame lost after ignition
- Carrier 59SC6A status code 14: flame-loss lockouts during a heat request
Codes are documented per controller; check the model number on your rating plate before comparing. Not sure which control you have?
Related
Questions people ask
Why does my furnace work after a reset if the sensor is dirty?
A marginal sensor sometimes proves flame on a later attempt, so resetting appears to help. The fouling is still there and the fault returns.
Is flame loss dangerous?
The control is closing the gas valve because it cannot prove flame, which is the safe behaviour. The danger would be gas flowing without proof of flame, which this safety prevents.
Not sure whether the flame sensor is your problem?
BC Wide diagnoses by testing, not by part swapping. Tell us the model number from the rating plate and what the furnace is doing.