Furnace Limit Switch Tripped: Symptoms, Causes

Quick Answer

A tripped limit switch is usually caused by restricted airflow from a dirty air filter or blocked return vents. Check your filter first and ensure all registers are open to allow the furnace to breathe and cool down properly.

I'd say seven out of ten limit switch calls I go on are just a dirty filter. That's it. But here's the thing, if you keep ignoring it, you're looking at a cracked heat exchanger, and that's not a $30 fix. That's a $1,500 repair or a new furnace conversation. The switch is doing exactly what it's supposed to do. Listen to it.

GenericFurnaceSeverity: highDifficulty: intermediate85% DIY Success

Time to Fix

15–60 min

Difficulty

intermediate

Parts Cost

—

Tools Needed

Phillips #2 screwdriver, Flat-head screwdriver, small, for spade connectors

What Does the LIMIT-TRIP Code Mean?

Nine times out of ten your furnace is just choking for air. The heat builds up, the limit switch hits its trip point, and everything shuts down. I've changed probably forty filters this month alone but replaced maybe three actual limit switches. Check the cheap stuff first. A $5 filter beats a $200 service call every single time.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Dirty Air Filter60%

Blocked Vents15%

Failing Blower Motor15%

Faulty Limit Switch10%

Symptoms You May Notice

The blower fan just keeps running and running after the burners shut off, sometimes 10 or 15 minutes, and the air coming from the vents feels cold or just barely room temperature.
Furnace fires up, runs for two or three minutes, then shuts off well before the house ever hits the thermostat setpoint.
A faint burning smell near the furnace cabinet, kind of like hot dust or scorched metal, especially on the first seriously cold days of the season.
The error code light on the control board is blinking a specific pattern - count the blinks and look for the legend sticker on the inside of the cabinet door, it'll tell you exactly what it means.
You hear the igniter click, the burners light, everything sounds normal for a minute or two, and then it all shuts down again like someone flipped a switch.

Can you reset a Generic furnace to clear the LIMIT-TRIP code?

Turn the thermostat down first so the furnace doesn't immediately try to fire back up. Flip the breaker off for a full 30 seconds. If yours has a manual reset button on the limit switch - it's a small raised button, usually red or white, sitting right between the two wire terminals on the switch body - press it until you feel it click. Let everything sit and cool for 10 to 15 minutes, then restore power and bump the thermostat up. If it trips again within the next hour, there's still an underlying cause you haven't tracked down yet.

Tools Required for Diagnosis

Phillips #2 screwdriverFlat-head screwdriver, small, for spade connectorsDigital multimeter with continuity functionNut driver set, 1/4 inch and 5/16 inchReplacement air filter, check size printed on old filter frameFlashlight or headlampReplacement limit switch if testing shows failure

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

ComponentComponent Under Test

Expected Range0–1 ohms

ConditionIf Open (OL) or infinite, replace component.

Frequently Asked Questions

What actually happens when a furnace limit switch trips?

The switch kills the 24V signal to the gas valve so the burners go out instantly. But the blower keeps running - that's actually intentional. The furnace is trying to dump all that stored heat out of the cabinet before things get worse. You'll feel cold air from the vents even though nothing's burning. Most of the time it'll reset itself once the exchanger cools down and then try to fire again. And it'll probably trip again if you haven't fixed the underlying cause.

Can I reset a furnace limit switch myself?

Yeah, usually. Modern furnaces typically have an auto-reset limit - once it cools to a safe temp the circuit closes on its own. But some older furnaces have a manual reset button on the switch body that needs a physical press. Either way, don't just keep resetting it over and over without figuring out why it's tripping. I've been on calls where the homeowner reset it six times in one day thinking the furnace was just having a moment. It wasn't - the blower capacitor was dying and had been for weeks.

Why does my limit switch keep tripping even with a new filter?

New filter is a great start but it's not always the whole story. Next thing I check is whether the evaporator coil is clogged - if you have central A/C, that coil sits right above the furnace and it accumulates dust all summer long with barely any airflow through it. It can choke the system just as badly as a dirty filter. After that I'm looking at the blower capacitor, then the motor itself. Sometimes the ductwork is just undersized for the furnace, especially if someone swapped in a bigger unit without upsizing the ducts to match.

Is it dangerous to run a furnace with a bad limit switch?

Really dangerous. If the switch is stuck closed - meaning it won't trip when it should - your furnace can overheat to the point of cracking the heat exchanger or melting internal wiring and components. And a cracked heat exchanger means combustion gases including carbon monoxide can get into your air supply. I've seen it happen. Never bypass the limit switch, not even for a quick test, unless you're literally standing there ready to kill power manually the second temps climb.

How do I test a limit switch with a multimeter?

Easy. Power off at the breaker first. Pull the two wire connectors off the switch terminals. Set your multimeter to Ohms or hit the continuity button. Touch a probe to each terminal. When the furnace is cold, a good switch should read basically zero Ohms - closed circuit, current can flow. If you're getting OL or a reading stuck at infinity, the switch has failed open and it won't let the gas valve fire even when temps are totally normal. Replace it. They're usually $15 to $25 at any HVAC supply house and take about ten minutes to swap out.

How long does a furnace limit switch last?

Honestly, a good one should last the life of the furnace, like 20 plus years. But if it's been tripping repeatedly because of a dirty filter or bad airflow, every trip stresses the bi-metal strip a little more. I've seen switches fail after a bad season where the homeowner kept resetting it instead of fixing the root cause. Think of it like a circuit breaker - trip it a hundred times and eventually the mechanism wears out. Fix the airflow problem first and the switch usually takes care of itself.

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 17, 2026