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How to Check the High Limit Switch on a Trane

Julie Boleware

The high limit switch (also called a main air limit switch) in a Trane furnace is a safety device that is supposed to be closed during normal operation. If the air temperature coming from the furnace gets too hot, the switch will open and force the furnace to shut down. There are several reasons why the air temperature will get too high, such as undersized ducts, a dirty evaporator coil, a dirty return air filter or a faulty high limit switch. You can check to see that your high limit switch is functioning properly by testing it.

Check the High Limit Switch on a Trane
  1. Look at the furnace date plate on the side of the air handler. This will show the maximum temperature that heated air should be when it enters a room through the supply vents.

  2. Block off the return air vent(s). Place a piece of cardboard over the top of the vent and tape it in place.

  3. Stick a temperature probe into the supply air plenum. The supply air plenum is the tube or duct that comes out of the top of the inside unit, also called the air handler.

  4. Turn on the heat.

  5. Wait for the temperature in the air plenum to reach the maximum supply air temperature. Listen to hear the furnace shut off the burners. Replace the high limit switch if the furnace does not shut off or get near the maximum supply air temperature.