Gas Furnace Flame Sensor Fix

By February 23, 2019 HVAC

Estimated reading time: 3 minutes, 7 seconds

Top half of a furnace

The flame sensor was an ingenious solution to a serious safety problem. On a gas furnace, they provide an incredible safety feature when they are working right. When they aren’t they become a menace and cause silly service calls.

For some reason there seems to be a correlation between furnaces near the laundry. One fellow suggested that a particular dryer sheets seem to be the culprit. I have not made that connection but who knows.

When the Furnace Just Wont Fire Up

Gas furnaces got through a series of safety checks before they come on. Some more than others. The life expectancy of a newer gas furnace is about 20 years but they have a functional obsolescence. In other words the technology is getting better all the time and they are designed to fail sooner so you get a new one with the new features.

Back in the day, a thermocouple was all you would get for a safety mechanism and a standing pilot. Now there are fans and suction pipes to verify operation, there are sensors for roll out, over temp, glow rods, flame sensors and fuses so there is no open flame. In the high performance units a condensation pump could shut your system down summer or winter if it fails.

What Does a Flame Sensor Do?

A flame sensor is a rod that sits in the flame to make sure that it is on shortly after the gas gets let go from the regulator. if no flame is sensed then the machine shuts off preventing gas from flowing into the machine and causing a build up of raw gas. If there is no flame then the computer shuts the furnace down for another start cycle. This will repeat for 3 times then it is locked out for a period of time.

Most furnaces have a LED light on a circuit board that you can read through a site glass. It flashes codes that you can correlate with a chart on the cover to discover what self diagnostic problem is detected.

What does the Flame Sensor Look Like?

Flame Sensor

It is typically a straight or bent at 90 degree rod, with 1 wire going to it that lives in the flame away from the ignighter or glow rod.

Fixing / Cleaning the Flame Sensor

An unethical service fellow might replace it.  Reality is that if you can remove it then use a gentile brushing with an abrasive cloth or green scrubby. You will get it back in fighting shape. You may or may not see any debris on the sensor, I have only replaced one failed unit after cleaning it annually twice out of frustration. They do not make it easy to get out or put back so you may not have the right stubby tools but it can be done. I have been the hero on many occasions by understanding how the thing works and how to fix it.

If you plan to test it with the covers off you will need to temporarily override the cover switch to see if it works.


Your comments are welcome. To ask questions or get more information about remodeling or fixing things, click here to email me directly, or call 208-639-1808

I do these things during non-Levco time to be sure it doesn’t interfere with the Remodeling business. Repairing things and understanding homes is just another passion of mine.

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Disclaimer: Some of these images came from the WEB. If they are yours, and you object to them being used, please claim them and I will gladly remove and replace them at once.

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