A furnace limit switch is a heat-sensing safety that shuts the burners off when the inside of the furnace gets too hot. When it trips, the gas shuts off, the blower keeps running to cool things down, and the burners relight once the temperature drops. If that happens over and over, the switch is almost always doing its job: something is choking the airflow, and the furnace is overheating. Replacing the switch without fixing the airflow fixes nothing.
What a Furnace Limit Switch Does
Inside, most limit switches use a bimetal disc that snaps open at a set temperature and closes again once it cools. The switch is wired into the safety circuit that powers the gas valve. While it's closed, the furnace can fire. When it opens, the control board closes the gas valve and runs the blower to pull heat away from the heat exchanger.
The opening temperature is set at the factory for that specific furnace and is usually stamped on the switch body. That number matters: the switch is matched to the heat exchanger it protects.
Types of furnace limit switches
Most guides talk about "the" limit switch, but a typical gas furnace can have several. Knowing which one tripped changes what it means.
| Switch | Where it sits | Reset | What a trip usually means |
|---|---|---|---|
| Primary high limit | On the wall between the burners and blower, probe reaching toward the heat exchanger or supply plenum | Automatic | Overheating, usually from restricted airflow |
| Auxiliary or secondary limit | Near the blower, common on downflow and horizontal furnaces | Automatic or manual, depending on model | Heat rising back into the blower, often when the blower stops or fails |
| Flame rollout switch | On the burner box, near the burners | Manual (small button) | Flame or heat escaping the burner area: blocked flue or heat exchanger, venting problem or a crack |
| Fan-limit control | Older furnaces only, a box with a dial on the supply plenum | Automatic | Combines the blower on/off control and the high limit in one device |
A rollout trip is a different category of problem from a high limit trip. If your furnace has a tripped rollout switch, treat it as a safety call for a technician, not a reset-and-forget event.
Symptoms of a Tripping or Bad Limit Switch
- The burner shuts off a few minutes into each cycle while the blower keeps running, then relights a few minutes later. This repeating pattern is the signature of a high limit trip.
- The house never reaches the thermostat setting even though the furnace seems to run constantly.
- Cool air at the vents in the gaps when the burner is off. See furnace blowing cold air for how to tell this apart from other causes.
- A limit code on the control board LED. The sticker inside the door decodes the blink count.
- A blower that won't shut off. If the switch has opened and not reset, many boards keep the blower running.
- No heat at all after the furnace has locked out from repeated trips.
Compare that with a dirty furnace flame sensor, which shuts the burner off within seconds of lighting, not minutes.
Why a High Limit Switch Keeps Tripping
Air carries heat away from the heat exchanger. Anything that slows that air lets the metal and the supply air get hotter until the switch opens. Work through these roughly in order of how often they're the cause.
- A clogged air filter. By far the most common. If you can't see light through it, replace it. See changing your furnace filter for sizes and schedules. A very restrictive high-MERV filter in a system not designed for it can cause the same thing.
- Closed or blocked supply registers. Closing vents in unused rooms raises pressure in the ducts and cuts total airflow.
- Blocked return grilles. Furniture, rugs or boxes over a return starve the blower.
- A dirty blower wheel. Dust packs into the blades of the squirrel cage and cuts its output.
- A weak blower motor or capacitor. A motor that runs slow moves less air. See furnace blower motor problems.
- A dirty AC evaporator coil. On most systems the cooling coil sits right on top of the furnace, so winter air passes through it too. A matted coil restricts heating airflow just like a dirty filter.
- Undersized or restricted ductwork. Common after a furnace replacement where a larger unit was matched to old ducts, or when ducts are crushed or disconnected. When the furnace is simply too big for the ducts, the fix is a duct or equipment change, and new furnace pricing explains how a properly sized swap is priced.
- Blower speed set too low for heating, sometimes after a repair or a control board replacement.
- Overfiring. Gas pressure set too high puts more heat into the heat exchanger than the airflow can remove.
- A worn switch. Limit switches can drift or fail with age, opening at a lower temperature than they should. This is real but less common than airflow, so it should be the conclusion after the airflow checks, not the starting assumption.
The Temperature Rise Check
Here's a test most limit switch articles leave out, and a homeowner can do it with a probe thermometer. Every gas furnace has a rating plate (usually inside the door) that lists a temperature rise range, such as "35 to 65 degrees F." That's the acceptable difference between the air going into the furnace and the air coming out.
- Replace the filter and open every register and return first.
- Run the furnace in heat for about 10 to 15 minutes so it reaches steady state.
- Measure the air temperature at a return grille near the furnace.
- Measure the supply air at the register closest to the furnace.
- Subtract the return reading from the supply reading.
If the rise is above the top of the range, the furnace isn't getting enough air for the heat it's making, and limit trips are expected. A technician can confirm with static pressure readings and find where the restriction is. If the rise is within range and the switch still trips, the switch itself, its wiring or an overfire condition become more likely suspects.
The register readings aren't as accurate as a technician's readings in the plenum, but they're good enough to show whether you're in the ballpark or way over.
How a Technician Tests a Limit Switch
With the power off, a technician checks the switch for continuity at room temperature. A cold switch that reads open has failed. With the furnace running, they can measure voltage across the switch to see exactly when it opens, and compare the temperature at that moment with the rating. They'll measure temperature rise and external static pressure to find the airflow problem behind the trips, and look at the heat exchanger for signs of overheating.
Replacing a Furnace Limit Switch
If the switch has truly failed, the replacement has to match the original's opening temperature, probe length and mounting. A switch with a higher temperature rating will let the furnace run hotter than it was designed for, which defeats the purpose. Use the part listed for your furnace's model number.
Trane's homeowner troubleshooting page puts limit switch replacement at roughly $100 to $375, depending on the furnace model, location and service call fees. A good technician will also tell you what was causing the trips; if the answer is "nothing, the switch was just bad," ask what the temperature rise was.
Why Repeated Trips Matter
A furnace that runs right up to its limit cycle after cycle is putting thermal stress on the heat exchanger every time. Repeated overheating is one of the recognized causes of a cracked heat exchanger, which can let combustion gases, including carbon monoxide, mix with the air in your house. That's the real reason to fix limit trips promptly even when the house still feels warm enough.
What Not to Attempt Yourself
- Never jumper or bypass a limit or rollout switch to keep the furnace running. It removes the protection against overheating and fire.
- Don't press a rollout reset more than once. A rollout trip means flame went where it shouldn't. If it trips again, turn the furnace off and have it inspected.
- Don't substitute a different temperature switch because it was the one the store had.
- Don't test switches with the furnace powered. The cabinet has 120-volt wiring next to the switch terminals.
- Stop if you smell gas or your CO alarm sounds. Leave the house and call from outside.
What you can do safely: replace the filter, open every register, clear returns, and run the temperature rise check. If the furnace keeps tripping after that, the furnace not working troubleshooting guide lists the next steps and what to tell the technician.
Frequently Asked Questions
How do I know if my furnace limit switch is bad?
A bad or tripping limit switch shows up as a burner that shuts off a few minutes into each cycle while the blower keeps running, a furnace that blows cool air and restarts later, or a limit code on the control board. Most of the time the switch is working correctly and reacting to overheating from poor airflow. A technician confirms a failed switch with a meter after ruling out airflow.
Can I reset a furnace limit switch?
Most primary high limit switches reset themselves once the furnace cools. Rollout switches and some auxiliary limits have a small manual reset button. Pressing a manual reset once is reasonable. If it trips again, stop and have the furnace inspected, because a rollout trip means flame or heat is escaping where it shouldn't.
What causes a high limit switch to keep tripping?
Restricted airflow is the usual cause: a clogged filter, closed or blocked registers, a blocked return, a dirty blower wheel, a weak blower motor or capacitor, a dirty AC coil above the furnace, or undersized ductwork. Less often it's a worn switch, an overfired burner or an oversized furnace.
Can I bypass a furnace limit switch?
No. The limit switch is what stops an overheating furnace from damaging the heat exchanger or starting a fire. Jumpering it removes that protection, and repeated overheating is a known cause of cracked heat exchangers, which can leak carbon monoxide.
Where is the limit switch on a furnace?
On most furnaces the primary limit is a small disc or probe-style switch mounted on the metal wall between the burners and the blower compartment, with its sensing end reaching toward the heat exchanger or supply plenum. Rollout switches sit on the burner box near the burners. Some furnaces have an extra auxiliary limit near the blower.
What's the difference between a limit switch and a rollout switch?
A high limit switch watches the temperature of the air or metal around the heat exchanger and usually resets itself. A rollout switch sits by the burners and trips if flames or heat roll out of the burner area, which points to a blocked flue, a cracked heat exchanger or another venting problem. Rollout switches normally need a manual reset and a technician's inspection.