How to test a furnace limit switch with a multimeter

If your furnace isn’t keeping you warm or keeps shutting off randomly, the limit switch might be acting up. Testing it yourself can save you some time and money, and it’s easier than you’d think. Let’s walk through how to check your furnace limit switch using a multimeter.

This info might not match your specific furnace model exactly. We aim to offer helpful advice for all DIY repairs, but different models can have variations in steps, error codes, parts, and so on. For the most accurate guidance, it’s a good idea to consult your manufacturer’s documentation or reach out to us with any questions.

What’s a furnace limit switch?

Before we get started, let’s quickly cover what the furnace limit switch does. It’s a safety device near your furnace’s heat exchanger. It keeps tabs on the temperature inside the furnace and shuts it off if things get too hot. This helps prevent overheating and potential damage.

What you’ll need

To test your furnace limit switch, grab these tools:

• A multimeter (one with a continuity setting works best)
• Screwdrivers or nut drivers  
• Your phone camera or a notepad to keep track of wire connections
• Optional: A hairdryer or heat gun to test with heat

Step-by-step guide to test your furnace limit switch

Always put safety at the top of your list when working with appliances. Before you do anything, find your furnace’s power switch or circuit breaker and turn it off. This prevents any electrical mistakes while you’re working.

Access the limit switch

1. You’ll need to find and access the limit switch inside your furnace. It’s usually near the heat exchanger, a small rectangular piece with wires attached. 
2. If you’re not sure, check your furnace’s manual or learn how to find your appliance’s model number.
3. Use a screwdriver or nut driver to take off the furnace’s access panel so you can see inside.
4. 3. Disconnect the limit switch. Take a photo or write down how the wires are connected first. This will help you put everything back correctly later.
5. Carefully remove the wires from the switch terminals.

Test with a multimeter

1. Set the multimeter dial to the continuity setting, usually shown by a sound wave or diode symbol. This lets you check if a circuit is complete.
2. If there’s no continuity setting, set it to measure resistance (ohms), shown by the Ω symbol.
3. Touch the multimeter probes to the terminals on the limit switch. It does not matter which probe goes where.
4. At room temp, the limit switch should be closed, meaning the circuit’s complete. Your multimeter should beep or show a continuous reading, indicating continuity. On a resistance setting it should display a low resistance value, close to zero ohms.

If it shows continuity, the switch is closed as it should be at room temp. If there’s no continuity (no beep or infinite resistance), the switch might be faulty.

Test the limit switch with heat  

1. To check if the switch opens when it gets hot, gently heat the limit switch with a hairdryer or heat gun. Be careful not to overdo it or cause damage.
2. As it warms up, the switch should open, breaking the circuit.
3. This change means the limit switch is doing its job, opening when it reaches its high temp setting (usually between 150°F and 200°F).

If you need a replacement, you can find the right part at AppliancePartsPros.com. Have your furnace’s model number handy; if you’re not sure where to find it, here’s how.

Reassembly

1. If you heated the switch, give it time to cool down.
2. Use your notes or photo to reconnect the wires properly.
3. Put the furnace’s access panel back on securely.
4. Restore power to the furnace and set your thermostat to call for heat.
5. Make sure the furnace operates normally without unexpected shutoffs.

Additional help

For more help with furnace repairs, swing by our repair help section. If you find out you need a new part or more help, we’re here for you.

Visit AppliancePartsPros.com for genuine replacement parts and expert advice on your furnace and other appliances. Stay connected with us on Facebook and Twitter for the latest tips, updates, and support.

If you’ve got any questions or need a hand, don’t hesitate to contact us. We’re here to make your DIY repair journey as smooth as possible.

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