How to Troubleshoot a Furnace Flame Sensor

If you have a furnace that is cycling on and off very quickly and it is fitted with a electronic thermal

couple or flame sensor, you may notice that not enough heat is building up to warming your home. To diagnose this problem, turn the furnace on and watch the flames light up. If the flame goes out quickly, the flame sensor may not be communicating to the gas supply correctly. The coil will heat up, ignite the flame and then the flame will go out very quickly without heating the air in the furnace. The problem is that the flame sensor isn't heating up properly, and you should remove it from the furnace to inspect it and see it debris is clogging the system and not allowing the furnace to warm your home.

Removing the Flame Sensor
To remove the flame sensor you'll loosen the screw on the top of the furnace over the second burner. Once the scree is loosened the bracket holding the flame sensor in place can be removed. Without detaching the flame sensor from the furnace, you can use a piece of emery cloth to sand the build up for the end of the flame sensor. The build up insulates the end of the flame sensor so it doesn't heat up properly. When the thermocouple doesn't get hot enough it shuts down the gas control as a safety measure.
 
Reattaching the Flame Sensor
Reattach the flame sensor by putting back into the hole on the top of the furnace and tighten the screw on the bracket holding it in place. Make sure to check that the sensor is in the same position that you found it before you removed it for cleaning. Typically it will be positioned just over the second burner. When the furnace is reignited the flame sensor will be touching the flame so it can heat up correctly. When the flame sensor is able to detect that the flame is lit the gas supply will stay on, keeping the flame fed and allowing for heat to build up and warm your house.

For a furnace that is cycling off too fast, or if you notice that your home isn't warming up a light sanding of the flame sensor may be your fix. You can add years of use to the life of the flame sensor just by giving it a light sanding once a season. If you are comfortable checking this item yourself, you can save money on a service call. But if you're not a DIY-er, contact a local HVAC Denver professional to come out and diagnose the problem.