How to Repair a Furnace Fan Limit Control Switch

Turn off the furnace at the thermostat and switch off the circuit breaker.

Take off the cover plate on the side of the furnace using a screwdriver to remove the screws. The fan limit switch is at the top on the right inside many furnace models. A dial control in the center of the unit is labeled "Fan Limit," and the part is connected by two wires.

Insert the tip of a flat-blade screwdriver into the slot above each wire connection and pull down on the screwdriver handle to release each wire from its terminal.

Twist the ends of the two wires together to make a temporary splice. Turn on the circuit breaker and thermostat, adjusting the temperature higher than the room temperature to activate the furnace. If the blower motor activates after a few minutes and did not activate before or would not shut off, the fan limit control switch is defective.

Turn off the thermostat and circuit breaker.

Use a screwdriver to remove the screws in the mounting holes on the limit control switch. Remove the part.

Hold the new switch over the mounting holes in the furnace and secure the part with screws.

Disconnect the two wires from each other and insert each wire into its respective terminal on the switch.

Slide the pointers on the switch dial to the settings recommended for your particular furnace. Pull the manual override control in the lower left corner to the "Off" position. This prevents the switch from directing the motor to run continuously.

Replace the cover panel with screws and switch on the furnace.