How to Test a Limit Switch on a Gas Furnace

The limit switch in a gas furnace prevents the blower motor from activating if the gas burner is still warming the heat exchanger. This typically occurs when the furnace is first turned on after a period of inactivity. The switch is in the open, or deactivated, position until the temperature reaches a certain level. This causes the switch to close and start the blower. Test the switch using a multimeter to verify the part does not have a short circuit.

Nut driver
Multimeter
Screwdriver

Instructions

- 1
  Shut off the furnace at the wall thermostat, then cut the power at the circuit breaker. Even though it is a gas furnace, the unit requires electricity to run the blower motor, start the igniter and operate other components.
- 2
  Remove the bolts in the service panel on the side of the furnace using a nut driver.
- 3
  Pull off the panel and set aside to locate the blower assembly, which typically resembles a metal canister with a fan motor adjacent to it. The limit switch is attached with two screws to the side of the blower. Two wires are connected to the switch contacts.
- 4
  Slide the connectors on the end of the wires off the switch contacts.
- 5
  Dial the multimeter to measure ohms and place the two rods on the end of the meter against the two switch contacts. A zero reading means the switch is operating normally in the open position. Any other reading while the furnace is shut off means the switch is defective and fused in the closed position.
- 6
  Remove the screws holding the switch to the blower canister using a screwdriver. Take the part to an HVAC supply or repair shop for a replacement.

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