The flame sensor in a furnace checks for heat or even heating from the burner. If the sensor detects a problem, it shuts off the burner after about 10 seconds. The furnace attempts to start again an additional two times, after which, the circuitry creates a 2-hour lockout.
The flame sensor is a safety feature of the furnace. If there is no flame, there is the potential for a buildup of explosive fuel in the home. An uneven flame increases the production of carbon monoxide, a toxic gas. Sometimes extinguishing the flame and starting again allows the problem to correct itself with no further action.
Flame sensors sometimes malfunction and stop the heat when there is not an issue with the quality of the flame. Rust and corrosion on the sensor lead to a faulty reading. They are easily removable using steel wool. Sensors sometimes become defective and require replacement. Cracked porcelain on the flame detector often indicates the need for replacement.
While the lockout is temporary, cleaning or replacing the sensor prepares the furnace for use again. If the furnace does not restart after the repairs, turning the switch to the furnace and then turning it on again resets the circuitry. The reset should allow the furnace to attempt starting again, restoring heat to the home.