Why does my furnace limit switch keep tripping?
Almost always because the furnace is overheating from too little airflow: a clogged filter, closed registers, a blocked return, a dirty AC coil above the furnace, or a weak blower. The limit switch is doing its job by shutting the burners off. Replacing the switch without fixing the airflow brings the problem right back.
Key points
- The high-limit switch shuts off the burners when the furnace gets too hot. It's a safety device, not usually the problem.
- Most of the time the cause is airflow: filter, registers, returns, the AC coil or the blower.
- Tripping looks like this: burners run a few minutes, shut off while the fan keeps blowing, then relight later.
- A tripped rollout switch is a different and more serious warning. Don't keep resetting it.
- National cost guides put a limit switch replacement at $100–$375, but the airflow fix is often the real repair.
A furnace limit switch that keeps tripping is almost always reporting an airflow problem: not enough air is moving across the heat exchanger, so the furnace runs hotter than it's designed to and the switch shuts the burners off. The switch itself is rarely the failed part. Fix the airflow (most often the filter, then the registers, returns, AC coil or blower) and the trips stop. It's one of the most common winter service calls in Minnesota, and one of the most misunderstood. Here we cover what the switch does, what a trip looks like from your side, what you can check yourself, and the order a technician works through the rest.
What does a furnace limit switch do?
Every gas furnace has a high-limit switch: a small temperature-sensing switch mounted where it can feel the heat coming off the heat exchanger. If the furnace gets hotter than it's designed to run, the switch opens, the control board shuts the gas valve, and the blower keeps running to cool things down. Once the temperature drops, the switch closes again and the furnace can try another cycle.
On older furnaces you may find a combination fan and limit control with a round dial and adjustable pointers. That control also decided when the blower came on. On most furnaces built in the last few decades, the control board runs the blower on timers and the limit switch is a fixed-temperature safety.
Many control boards count limit trips. After several in a row, the board may lock the furnace out for a while, which is why a limit problem can look like a dead furnace.
Two other safety switches get lumped in with the limit, and they mean different things:
- Rollout switches sit near the burners. They trip when flame rolls out of the burner area instead of going where it should. That can point to a blocked vent, a blocked heat exchanger or a cracked one. Rollout switches are usually manual reset. If one trips, don't keep resetting it. Shut the furnace off and call.
- Auxiliary limits, found in the blower area on some furnaces, protect against overheating when the blower isn't moving air. Some are manual reset too.
What does a furnace limit trip look like?
- The burners light and run for a few minutes, then shut off while the blower keeps going.
- A few minutes later the furnace relights and does it again.
- The house never quite reaches the thermostat setting, and the air at the registers feels warm but not hot.
- The status light on the control board flashes a code that the chart on the furnace door lists as an open limit or high limit.
That on-off pattern is also what people call short cycling, which has other causes as well. Our furnace short cycling guide covers the rest of that list. When the pattern is a limit trip, this table matches what you're seeing to the likely cause:
| What you notice | Likely cause | What to do | Who does it |
|---|---|---|---|
| Trips started right after a filter change | New filter too restrictive for a 1-inch slot | Go back to a lower-MERV filter, or add a media cabinet | You, then a technician for the cabinet |
| Filter is gray and caked | Clogged filter | Replace it | You |
| Trips began after closing registers or finishing the basement | Too little supply or return air | Open registers, uncover or add returns | You, then a technician |
| Weak air at every register, blower sounds slow | Weak capacitor, failing motor, or a packed blower wheel | Test, clean or replace | Technician |
| Trips got worse over a few seasons | Dirty AC coil on top of the furnace | Clean the coil, measure static pressure | Technician |
| Trips on a newly installed furnace | Heating blower speed or gas pressure set wrong | Recheck setup against the rating plate | Technician |
| A switch near the burners needs a button pressed | Rollout switch, not the limit | Stop and call; don't reset it | Technician |
Check these first
Most limit trips come down to air not getting through the furnace fast enough to carry the heat away. Before you call, check:
- The filter. Pull it and hold it up to a light. If you can't see through it, replace it. If you recently switched to a high-MERV 1-inch filter and the trips started then, that filter may be too restrictive for your system. Our MERV rating guide explains the trade-off.
- Supply registers. Open them. Closing registers in unused rooms raises the pressure the blower works against.
- Return grilles. Move furniture, rugs and boxes off them.
- The blower door. Make sure the panel is seated. Many furnaces won't run right with it loose.
- One reset. Turn off the furnace power switch for about 30 seconds and back on. If it trips again within a cycle or two, stop and call.
What a technician checks, in order
When a filter change doesn't fix it, this is the order we usually work in on a furnace repair call:
- Temperature rise. We measure the air temperature coming off the furnace and going into it. The rating plate lists an allowed range. Above that range means too little airflow or too much fire.
- Static pressure. How hard the blower is pushing against the ducts, compared with what the furnace is designed for.
- The blower wheel. Dust, lint and pet hair packed into the fins cut airflow sharply and are easy to miss.
- The blower motor and capacitor. A weak run capacitor on an older PSC motor slows the blower. A failing motor does the same. Our blower motor replacement cost guide covers that repair.
- The AC coil. The air-conditioning coil sits right on top of most furnaces, so a dirty coil blocks heating airflow in January too.
- Blower speed settings and gas pressure. A heating speed set too low, or a furnace firing harder than its rating, both run it hot.
- The switch itself. A limit switch can weaken with age and open below its rated temperature. We test it before replacing it.
If the furnace has been overheating for a long time, we also look closely at the heat exchanger. Our post on cracked heat exchanger symptoms explains what we're looking for.
Why do limit trips pile up in a Minnesota winter?
The same furnace that ran fine in October can start tripping in January:
- Cold snaps mean long cycles. At -15°F a furnace may run nearly nonstop, and a marginal airflow problem that stayed hidden on short fall cycles finally pushes it over the limit.
- Filters load up fast once the house is closed and the blower runs for hours a day, and a high-MERV 1-inch filter bought for allergies or a smoky summer restricts air even when clean.
- The AC coil stays in the airstream all winter, carrying whatever dust got past the filter last summer.
- Older houses on original ducts. Many Minneapolis and Saint Paul furnaces replaced gravity heat decades ago and still run on those ducts, and finished basements often cover up the returns. Our page on furnace repair in Saint Paul covers those houses.
If the heat quits on a subzero night, our checklist for no heat in a Minnesota winter walks through the safe checks and how to protect pipes while you wait.
When should you stop resetting the furnace and call?
Stop and call if any of these happen:
- The furnace trips again after a new filter, open registers and one reset.
- A rollout switch has tripped, or you see scorch marks or soot around the burner area.
- You smell burning, hot plastic or anything electrical.
- The board has locked the furnace out and the house is losing heat in subzero weather.
- A carbon monoxide alarm sounds. Get outside first and call 911.
How much does a furnace limit switch replacement cost?
National cost guides like HomeGuide put a limit switch replacement at $100–$375. If the real problem is the blower, HomeGuide's typical national ranges are $500–$1,400 for a PSC motor and $800–$2,300 for an ECM variable-speed motor. Filters, a cleaned blower wheel or opened registers cost far less. Our furnace repair cost guide covers the other common repairs.
A service call carries a $79 trip charge that's waived when you approve the repair, and you get a written, flat-rate price before any work starts. Completed repairs carry our written 1-year parts-and-labor warranty.
What to do next
Change the filter and open the registers first. If the furnace still cycles off on limit, call us at 612-429-3465, open 7 days, 7am–9pm, with no-heat calls getting priority. A fall tune-up is also a good time to catch a dirty blower wheel or a weak capacitor before the first cold snap does, and you can ask us about a maintenance plan if you would rather not keep track of it yourself.
Questions
Can I bypass or jumper a furnace limit switch to get heat back?
No. The limit switch is what stops the furnace from overheating its heat exchanger. Jumping it out removes that protection and can crack the heat exchanger or start a fire. If the furnace is tripping on limit, shut it down, check the filter and registers, and call for service. Space heaters on a separate circuit are the safer stopgap.
How do I reset a furnace limit switch?
The main high-limit switch resets itself once the furnace cools. What you can reset is the control board: turn off the furnace's power switch for about 30 seconds and back on. If the furnace trips again within a cycle or two, stop resetting it. Some furnaces also have a manual-reset limit or rollout switch, and a tripped one needs a technician to find out why.
Can a dirty filter really trip the limit switch?
Yes, and it's the most common cause we find. A clogged filter starves the furnace of return air, the heat exchanger runs hotter than it should, and the limit opens. A brand-new filter that's too restrictive for a 1-inch slot can do the same thing. If a filter change fixes it, keep changing it on a schedule.
Is it dangerous if my limit switch keeps tripping?
Not in the moment, because the switch is doing its job, but the overheating behind it isn't harmless. Heat exchangers are built to heat up and cool down, not to overheat over and over, and repeated overheating is one of the stresses that can lead to cracks. Make sure your CO alarms work, and get the airflow problem found rather than living with the cycling.
Sources
- HomeGuide cost guides (2026 editions) homeguide.com