How to test the furnace limit switc.
If the high limit switch was bad, the furnace should not fire. When the high limit switch trips, the flame/heating element will be shut off and the blower will continue to run to clear the heat. The blower will continue to run until either the system is reset, or the limit switch closes.
I believe high limit switches are normally closed, and they should always fail open. Which means when they go bad, or the temperature gets too high they open the circuit. To test a limit switch, you'll want to check continuity to determine what state the switch is in.
- If there is a serviceman switch, turn it off. If not, shut the power off at the breaker panel.
- Open the access panel and locate the high limit switch. It should be somewhere on the hot box.
- Disconnect the wires from the switch (don't forget which goes where).
- Using a multimeter, test for continuity between the leads on the switch.
If the circuit is open, you'll want to replace the limit switch.
If the limit switch is adjustable, make sure it is set properly according to the manufactures specifications.
If the limit switch tests OK; or you've replaced it and still have the issue, you'll want to focus on the thermostat.