How to Check the High-Limit Switch Bypass on the Furnace

A furnace high-limit switch controls the blower motor. The switch lets the motor run for up to 90 seconds after the desired temperature is reached in the building. This causes the heat exchanger to cool gently and to prevent overheating, which can damage the unit. If the furnace runs continuously or seems to be operating irregularly, bypassing the high-limit switch is a common troubleshooting technique to determine if the switch is defective. Bypassing the switch is not a permanent solution, only a diagnostic test.

Nut driver set
Electrician's tape
Scissors

Instructions

- 1
  Shut off the furnace at the circuit breaker.
- 2
  Remove the hardware holding the service panel to the furnace cabinet using a nut driver. The high-limit switch is near the top of the cabinet, below an angular draft hood. A metal probe up to 6 inches long extends from the switch into the heat exchanger. This is a temperature sensor wired to the switch.
- 3
  Disconnect the two wires attached to the high-limit switch. Each pulls off a thin metal blade.
- 4
  Connect the wires together and splice with a piece of tape, using scissors to cut the tape.
- 5
  Turn on the furnace and set the wall thermostat to the desired temperature. If the furnace blower switches on immediately, or if it continues running for more than five minutes after the desired room temperature is reached, the switch is defective.

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