A flame sensor is designed to regulate the flow of gas from a gas valve, much like a thermocouple. Testing one is essential to maintaining proper and safe functionality of a gas-powered device regulated by a flame sensor, such as a gas dryer, furnace or water boiler. A malfunctioning flame sensor could cause permanent damage to the device, irregular function and a serious fire. Fortunately testing a flame sensor is straightforward, requiring just a multimeter and a light source.
Locate the flame sensor on the gas-powered device; it's usually found inside a small back or front access panel.
Unplug the device or, if the device cannot be unplugged, switch the breaker inside the breaker box responsible for the device to the "Off" position.
Unfasten the clamp securing the flame sensor in place using a Phillips screwdriver and disconnect the leads by pulling on the wire boots.
Set the multimeter to show resistance and touch the probes to the blue and white wire ports on the flame sensor; the multimeter should show low resistance.
Press the open end of the flame sensor against a 60-watt light bulb or a flame sensor test lamp. Inspect the multimeter; it should now show high resistance.
Replace the flame sensor if the multimeter doesn't show any discrepancy in resistance when the open end of the flame sensor is placed against a light source.