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Douglas Cooling & Heating Blog

Why Does My Furnace’s High Limit Switch Keep Tripping?

Your furnace has many parts that must function correctly in order to heat your home the way you like. When components malfunction, the system can lose efficiency or fail to operate entirely. One common problem that homeowners across the Birmingham, AL, area sometimes experience is a furnace high limit switch that frequently trips. Douglas Cooling & Heating explains what high limit switches do, why this issue occurs, and how to fix it.

What Is a Furnace Limit Switch?

The high limit switch is a furnace component that controls elements of operation and safeguards the system. This component is connected to the blower motor as well as the gas valve and consists of a probe that detects temperatures mounted to a metal plate.

The limit switch has two functions.

  • First, the limit switch tells the blower fan when to turn on and off. It does so by sensing air supply temperature, telling the fan to turn on once air is heated in order to circulate it into the home. It allows the blower to run even after the furnace has turned off its burners, so all heated air is distributed into the living areas.
  • Second, the limit switch detects the supply air temperature in order to identify overheating. If the supply air is too hot, the interior of the furnace overheats, which can cause damage to components. The limit switch trips when temperatures are too hot, effectively shutting down the system and locking furnace operation until the unit cools.

Why a Furnace High Limit Switch Trips

You’ll realize that your furnace high limit switch is tripping because heating cycles will end shortly after they begin. This issue is called short cycling, and it’s known to cause excess energy consumption and wear out your furnace faster. 

When the furnace high limit switch trips, there are issues within the system that must be resolved in order to allow the limit switch to operate properly.

Dirty Flame Sensor

The high limit switch has a flame sensor rod that detects air temperatures in the plenum of the furnace. If it is dirty, it cannot properly detect temperatures, which is one reason why the furnace high limit switch trips.

To clean the flame sensor rod, carefully follow these directions:

  1. Remove the furnace access panel to locate the high limit switch on the outside of the plenum.
  2. Unscrew the switch from the plenum.
  3. Use steel wool or sandpaper to gently clean away soot and other materials stuck on the rod.
  4. Replace the switch and access panel.

If it is not possible to clean the dirty flame sensor, it can be replaced. Contact your HVAC technician to replace this part correctly.


If the air inside your furnace gets too hot, the limit switch trips to prevent overheating. If the furnace high limit switch trips due to overheating, there is likely an airflow issue that is preventing proper air movement through the system.

  • Check your air filter, and replace it if dirty. A dirty filter is the most common cause of airflow issues.
  • Open all vents and registers throughout the home to ensure proper air movement through the system and ducts.

A dirty blower motor can also cause overheating, which is one reason why the furnace high limit switch continues to trip repeatedly. Schedule a maintenance tune up to have the entire system and the blower motor thoroughly cleaned.

Defective Limit Switch

If the furnace high limit switch is tripping, it will wear out at some point. Eventually, it will trigger a lockout mode, and your gas furnace will be unable to operate until the switch is reset or replaced. Make sure you contact your technician to perform this repair task.

Hire Douglas Cooling & Heating for Furnace Repair Services

If your gas furnace high limit switch keeps tripping, call Douglas Cooling & Heating for fast furnace repairs. Our technicians find the source of the problem and make accurate repairs to ensure your switch doesn’t trip and your furnace operates as it should.

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