How to Troubleshoot a Gas Furnace Auxiliary Limit Switch


Many modern gas furnaces are equipped with a variety of internal diagnostics and automatic safety features that help diagnose and prevent potentially dangerous problems with the operation of the furnace. One feature found in many gas furnaces is an auxiliary limit switch. This switch is tripped when the blower motor fails to engage. If your auxiliary limit switch has been tripped, troubleshooting the switch can be done easily and quickly.

  • Locate the auxiliary limit switch on your furnace. The exact location will vary by model, but it will be somewhere on the blower housing. Refer to your owner's manual if necessary.

  • Reset the switch by pushing the reset button on the switch.

  • Set the thermostat to "call for heat" to test the furnace. If the switch trips again, then your blower motor is likely not operating and a service call is necessary.

Tips & Warnings

  • Do not continue to run your furnace if the auxiliary switch has been tripped, as it can be dangerous.

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