Replace the Tripped High-Limit Switch

If your multimeter confirmed an open loop (infinite resistance) across the terminals of the high-limit switch while the furnace was cool, the switch has failed and must be replaced to restore heating functionality.

1. Turn off the power to the furnace at the service switch or the circuit breaker panel to ensure safety.
2. Remove the furnace access door panel to locate the high-limit switch, typically mounted on the furnace blower housing or near the heat exchanger.
3. Disconnect the electrical wires attached to the switch terminals.
4. Unscrew the mounting screws or release the retaining clips holding the switch in place and remove the faulty unit.
5. Install the new high-limit switch, ensuring it is the exact manufacturer-specified replacement part, and secure it with the mounting screws or clips.
6. Reconnect the electrical wires to the terminals of the new switch.
7. Replace the furnace access door panel and restore power to the unit.
8. Set the thermostat to heat mode and verify that the burners ignite and the system operates normally.

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