Furnace high limit switch: what it does and why it trips
It is a safety device. When it keeps shutting your furnace off, it is telling you something is wrong with airflow.
The furnace high limit switch is a temperature sensor that turns the burners off if the furnace gets too hot. It protects the heat exchanger from cracking and lowers the risk of fire. When it trips again and again, the switch is rarely the real problem. Something is stopping heat from being carried away, most often poor airflow.
What the switch does
The switch sits near the heat exchanger and watches the temperature inside the furnace. If it climbs past a set limit, the switch opens and the gas valve closes. The blower usually keeps running to cool things down. Once the temperature drops, the switch closes and the furnace tries again.
Why it keeps tripping
Think of it as a smoke alarm for heat. You do not fix the alarm; you find out what set it off.
| Likely cause | Why it overheats the furnace |
|---|---|
| Dirty or clogged air filter | Less air crosses the heat exchanger |
| Blocked or closed vents and returns | Air has nowhere to go |
| Failing blower motor | Not enough air is moved |
| Dirty heat exchanger or blower wheel | Restricted airflow inside the unit |
| Undersized or crushed ductwork | Pressure builds up in the system |
| Faulty switch | Trips below the real limit; this is less common |
The pattern you will notice
The furnace lights, runs for a few minutes, then the burners go out while the fan keeps blowing. A minute or two later it starts again. If that loop sounds familiar, the high limit switch is probably doing its job.
What you can check yourself
- Replace the air filter if it is dirty.
- Open every supply vent and make sure return grilles are clear.
- Check that nothing is stacked against the furnace.
- Listen to the fan. A weak or noisy blower is worth reporting.
If the cycling stops after a new filter, you found the cause. If it does not, stop there.
Never bypass the switch
Some people suggest jumping the switch to keep the heat on. That removes the one thing preventing a damaged heat exchanger, which can leak carbon monoxide into the house. It is also electrical work, so it is not a DIY job.
- Do not remove panels or test the switch while the power is on.
- Do not open or test the blower capacitor.
- If you smell gas or a CO alarm sounds, leave and call your gas utility or 911 from outside.
Our advice: fix the airflow, then call if it returns
One trip after a very dirty filter is normal. A furnace that cycles on the limit more than once or twice after you fixed the filter needs a licensed HVAC technician. Tell them how long the burners run before shutting off. That timing helps them tell a restricted duct from a failing motor.
Frequently asked questions
What does a furnace high limit switch do?
It shuts off the burners when the temperature inside the furnace gets too high, protecting the heat exchanger and reducing fire risk.
Why does my furnace keep shutting off on the high limit?
Usually restricted airflow: a dirty filter, blocked vents, or a weak blower motor. A faulty switch is possible but less common.
Is it safe to bypass the high limit switch?
No. It is the main protection against overheating and a cracked heat exchanger, which can leak carbon monoxide. Leave it to a licensed HVAC technician.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Furnaces
- CDC. Carbon Monoxide Poisoning Basics