When a furnace flame sensor fails, it can prevent the furnace from igniting or cause it to flicker and shut off prematurely. The flame sensor is a safety device that detects the presence of a flame and sends a signal to the control board to keep the gas valve open. A malfunctioning flame sensor can lead to incomplete combustion, gas leaks, and other safety hazards. Common causes of flame sensor issues include dirt or debris buildup, corrosion, or damage to the sensor itself. Troubleshooting these issues requires inspecting the flame sensor, cleaning it if necessary, and replacing it if it is damaged.