How to test and replace a furnace limit switch

Having trouble with your furnace and think the limit switch might be the issue? Replacing a furnace limit switch is something you can handle with a little help. We’ll walk you through the steps so your furnace stays safe and runs smoothly.

The info here might not match your specific furnace model exactly. We aim to provide useful tips for all sorts of DIY repairs, but different models can vary in troubleshooting, error codes, parts, etc. For the best advice, we recommend checking your model’s manual, or feel free to reach out to us if you have questions.

What is the furnace limit switch?

Your furnace’s limit switch plays a key role in keeping things safe by keeping an eye on the temperature inside. It stops the system from overheating by shutting off the burner or heating element if things get too hot. If the switch isn’t working right, it might stop the furnace from running or fail to shut it down when it should, which can be a safety concern.

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How to test the limit switch

Before starting replacement, it’s a good idea to test the limit switch to make sure it’s causing the issue. Here’s what you can do:

1. Check for airflow issues

Sometimes, the limit switch might trip because the furnace is overheating due to poor airflow:

1. Make sure all the warm air registers in the house are open and not blocked.
2. Check that return vents aren’t covered up by furniture, rugs, or curtains.

2. Inspect the limit switch

The limit switch is typically near the top of the furnace burner assembly. Once you’ve located it:

1. Look over the switch for any signs of damage like cracks or rust.
2. Make sure it’s clean and not covered in dust or grime. If it is, give it a gentle wipe with a soft cloth.

3. Perform an operational test

To check if the limit switch is working when the furnace is running:

1. Start up your furnace and set the thermostat higher than the room temperature.
2. Let the furnace start and run for a bit.
3. After it has been running for a few minutes, turn the thermostat down.
4. Wait 3 to 5 seconds and listen for a click.

If you hear the click, the limit switch is telling the furnace to shut off the burners or heating element and is working correctly.

4. Conduct a continuity test

If you have a multimeter, you can do a continuity test on the limit switch:

1. Make sure the furnace is completely powered off to avoid any shocks.
2. Set your multimeter to the continuity setting. If you’re not sure how to use a multimeter, check out some resources on our repair help page.
3. Carefully unplug the wires connected to the limit switch terminals.
4. Touch the multimeter probes to each terminal on the switch.
5. If the multimeter beeps or shows continuity, the switch is good. If not, the switch needs to be replaced. 

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Safety precautions

Safety should always be a priority when working on any appliance. Here are a few practical tips to help you stay safe:

• Turn off the power to the appliance by unplugging it or switching off the circuit breaker before starting any maintenance or repairs. This protects you from the risk of electric shock.
• Wear insulated gloves to shield your hands from sharp edges, pinching hazards, or debris.
• Work at a steady pace and take your time to avoid accidents or injuries.
• Make sure your workspace is well-lit so you can clearly see and access the parts you’re working on.
• Keep the area clear of clutter, and ensure children and pets stay out of the workspace.
• Never handle internal parts with wet hands. Check that both your hands and the work area are completely dry.
• Refer to your appliance’s user manual for any specific instructions or safety guidelines related to the part or repair.
• Handle parts gently to avoid damage to the appliance or potential injuries from applying too much force.
• Use safety glasses when dealing with chemicals, dust, or large debris to protect your eyes from harm.
• If the appliance was recently used, give it enough time to cool down before working on any heated components.
• Take photos or notes of wiring connections and terminals before disconnecting anything to make reassembly easier.
• Avoid touching exposed wires or terminals. If you must handle wires, use non-conductive tools or wear insulated gloves to minimize the risk of electrical shock.

How to replace the furnace limit switch

If you’ve confirmed the switch is bad, here’s how you can replace it:

1. Start by shutting off the furnace using the thermostat.
2. Flip the circuit breaker that powers the furnace to ensure safety during the repair.
3. Find the front access panel on your furnace. Remove any screws or knobs securing the panel, then gently take the cover off and set it aside in a safe spot.
4. The limit switch is usually near the top of the burner assembly. It might be a small rectangle or circle, with wires connected to it.
5. If there’s a reset button that’s popped out, try pressing it back in. This might resolve the issue without needing a replacement.
6. Take a quick photo to note where each wire is connected for easy reconnection later. Disconnect the wires from the old switch.
7. Remove any screws that are securing the switch in place and remove it.
8. Place the new limit switch in the same spot as the old one. Secure it with screws.
9. Reconnect the wires to the correct terminals, using the photo you took to ensure accuracy.
10. Put the access panel cover back on the furnace. Screw it back into place or tighten the knobs.
11. Flip the circuit breaker back on. Turn the furnace on at the thermostat and set the temperature higher than room temperature.
12. Let the furnace run for a few minutes. Turn the thermostat down and listen for the click of the limit switch to confirm it’s working properly.

Pro Tip: Before putting in the new switch, make sure it’s the right one for your model. If you’re not sure, check out our guide on finding your appliance’s model number to get the correct part.

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We’re here to help

Fixing your furnace might sound tough, but with some guidance, you can keep your home cozy and safe. If you need help finding the right parts or have questions, feel free to reach out to us at AppliancePartsPros.com. We’re here to give you the support you need for all your DIY repair projects.

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If you’ve got questions or need more help, please contact us. We’re always here to help you keep your appliances working great.