Many furnaces have a flame sensor. The flame sensor is a safety mechanism that your furnace uses to detect if there's a flame or not. Understanding what it does and how you can fix it can help you keep your furnace working for you while avoiding costly repair bills.

The Flame Sensor…Senses Flames

The flame sensor on your furnace detects the presence of a flame through the process of flame rectification. When it senses a flame, it signals the control board that it's okay to leave the flame on. When it does not sense a flame, it sends a message to shut everything down.

When the Flame Sensor Isn't Sensing Flames

Usually, the flame sensor will send the signal to the control board and the control board will warn you there's a problem. Usually, the warning comes in the form of an Electronic Pilot Relight Warning, or EPRL alarm. That alarm typically takes the form a steady or blinking red LED on the control board.

In addition, there are some scenarios that can point to a flame sensor problem.

  • Burner starts but turns off very quickly
  • Furnace cycles on and off two or three times then shuts down

In either of these cases, it's possible that you have a flame sensor problem.

Dealing with a Flame Sensor Issue

The good news is the flame sensor can only have one of two problems. Either it's broken, or it's dirty. The odds of it being broken are very slim, but it does happen. You can tell if it's broken if the ceramic insulator at the end of the flame rod has a crack.

However, in most cases, a dirty flame sensor causes these issues. Over time, the flame sensor can build up a coating of rust and dirt. That build up insulates the flame rod, which dulls its ability to sense the flame. So it will always say there's no flame, even if there is one.

By cleaning the flame rod, you can restore it to health.

  • Turn off electricity to the furnace
  • Locate the flame sensor and unscrew (usually a rod close to the burner)
  • Use steel wool, fine sand paper, or emery cloth to clean and polish it
  • Screw it back in, turn the gas and electric back on

Sometimes the furnace design doesn't make for easy rod removal.  You can still clean it without having to remove it. When you have your regular inspection and cleaning, the technician should see to the cleaning the flame rod. So it's not something that you should have to do yourself unless you are between routine maintenance visits.

If you still have issues after you clean the flame sensor, then you should call a furnace repair service such as Maryland Oil Company immediately. If you don't feel like you're up to the task of fiddling around with your flame sensor, then you should call in a professional for that as well.