A flame sensor is a thin metal rod positioned in a gas furnace's burner flame that confirms combustion is actually happening. It carries a tiny electrical current through the flame to the control board; if no current is detected within seconds of ignition, the board shuts off the gas valve to prevent unburned gas from building up.
Why it matters to you
A dirty flame sensor is one of the most common causes of no-heat calls on cold Fraser Valley mornings: the burner lights, runs a few seconds, then shuts off. It is a safety feature working correctly, and usually an inexpensive fix.
Practical guidance
If your furnace lights and then goes out after a few seconds, note how many times it retries before locking out and tell the technician. Cleaning the sensor is part of a proper annual tune-up; replacing it is warranted when cleaning no longer holds.