Furnace High Limit Switch Keeps Tripping: Causes and Fixes

If your furnace keeps shutting off before your home reaches the set temperature, then starts back up a few minutes later only to shut off again, the high limit switch is likely tripping. This safety device prevents your furnace from overheating — and when it trips repeatedly, it's telling you something is wrong. Ignoring it can lead to a cracked heat exchanger, which is a $1,500–$3,000 repair or a full replacement.

At Furnace Direct, we sell Goodman furnaces at factory-direct pricing. Understanding limit switch problems helps you diagnose issues early, avoid costly damage, and know when it's time to call a pro versus when it's time to replace.

What the High Limit Switch Does

The high limit switch is a temperature-activated safety device mounted on the furnace's heat exchanger or supply air plenum. It monitors the air temperature leaving the heat exchanger. If the temperature exceeds a safe threshold (typically 150–200°F depending on the furnace model), the limit switch opens the circuit and shuts off the gas burners to prevent overheating.

Most limit switches are automatic-reset — meaning they close the circuit again once the temperature drops to a safe level. This is why you see the furnace cycle on and off repeatedly (short-cycling) when the limit switch is tripping. The burners fire, the temperature rises too high, the switch trips and shuts off the burners, the temperature drops, the switch resets, the burners fire again — and the cycle repeats.

The 7 Most Common Causes

1. Dirty Air Filter (Most Common)

A clogged air filter restricts airflow through the heat exchanger. With reduced airflow, the heat has nowhere to go — it builds up inside the heat exchanger until the limit switch trips. This is by far the most common cause and the easiest to fix.

Fix: Replace the air filter. If the filter is completely clogged, the limit switch may have been tripping for a while, which means the heat exchanger has been stressed by repeated overheating cycles. Check the filter monthly and replace it every 1–3 months.

2. Blocked or Closed Supply Registers

If too many supply registers in your home are closed (or blocked by furniture, rugs, or curtains), the airflow restriction has the same effect as a dirty filter — heat builds up in the furnace because there's nowhere for the warm air to go.

Fix: Open all supply registers and make sure none are blocked. As a rule, never close more than 20% of your home's registers — and ideally, keep them all open.

3. Blower Motor Failure or Malfunction

If the blower motor is failing, running at the wrong speed, or not starting at all, airflow across the heat exchanger is insufficient. Common blower issues include a failing capacitor (motor hums but doesn't spin or spins slowly), worn motor bearings (grinding noise, reduced speed), incorrect speed setting (installed on wrong speed tap), and a completely dead motor (no airflow at all).

Fix: Listen to the blower when the furnace fires. If you don't hear it start within 30–60 seconds of the burners lighting, or if it sounds sluggish or makes unusual noises, the blower motor or capacitor likely needs service.

4. Ductwork Problems

Major duct restrictions can cause the same symptoms as a dirty filter — reduced airflow leading to overheating. Common ductwork issues include collapsed or crushed flex duct in the attic or crawlspace, disconnected duct sections (the air goes nowhere useful), severely undersized ductwork for the furnace's BTU output, and excessive duct length or too many turns creating high static pressure.

Fix: Inspect accessible ductwork for obvious problems. A technician can measure static pressure to quantify the restriction.

5. Dirty or Failing Blower Wheel

Over time, the blower wheel (the squirrel-cage fan inside the blower housing) can accumulate dust, dirt, and debris on its blades. A dirty blower wheel moves less air — like a fan with dirty blades. In severe cases, the buildup is so thick that airflow drops significantly.

Fix: Clean the blower wheel. This involves removing the blower assembly and cleaning each blade. It's a dirty job but not technically difficult. A professional will do this during a thorough annual tune-up.

6. Oversized Furnace

If your furnace is too large for your home's ductwork and heating load, it produces more heat than the duct system can distribute. The excess heat builds up in the furnace, tripping the limit switch. This is a design problem — no amount of filter changes or blower adjustments will fix an oversized furnace on undersized ducts.

Fix: A two-stage or modulating furnace can partially compensate by running on low fire most of the time. A ductwork evaluation and potential modification is the proper long-term fix. In some cases, replacing with a correctly-sized furnace is the best option.

7. Faulty Limit Switch

Limit switches do wear out over time, especially after years of repeated tripping cycles. A faulty switch may trip at too low a temperature or fail to reset properly.

Fix: A technician can test the limit switch with a multimeter and compare its actual trip point to its rated temperature. Replacement limit switches cost $15–$50 for the part. However, always investigate WHY the switch was tripping before assuming the switch itself is bad — replacing the switch without fixing the root cause just removes the safety protection.

How to Test the Limit Switch

If you're comfortable with basic electrical testing:

1. Turn off furnace power
2. Locate the limit switch: It's a disc-shaped or cylindrical device mounted on the heat exchanger or supply plenum, with two wires connected to it
3. Disconnect one wire from the switch
4. Set multimeter to continuity or resistance
5. Test across the two switch terminals: When the switch is cool, you should read continuity (near zero ohms). If you read open (infinite resistance) when the switch is cool, it's stuck open and needs replacement.
6. Note the rated temperature: Printed on the switch body. This tells you the trip point (e.g., L180 means it opens at 180°F).

The Danger of Bypassing the Limit Switch

We've seen homeowners and even some technicians bypass a tripping limit switch by jumping the wires. Never do this. The limit switch exists to prevent your heat exchanger from overheating and cracking. A cracked heat exchanger leaks carbon monoxide into your home's air supply. Bypassing the limit switch removes the one safety device protecting you from this potentially fatal failure.

If the limit switch is tripping, find and fix the cause. Don't disable the safety device.

When It's Time to Replace the Furnace

If your furnace is experiencing repeated limit switch trips and the investigation reveals an oversized furnace, severely degraded ductwork that's impractical to fix, or a heat exchanger that's already showing signs of stress (cracks, corrosion, soot buildup), it may be more cost-effective to replace the furnace than to chase recurring problems on an aging system.

A properly-sized Goodman furnace from Furnace Direct — matched to your home's actual heating load — eliminates the oversizing issue and starts fresh with a new heat exchanger, new blower motor, and full factory warranty. At factory-direct pricing, the cost is significantly less than what a contractor would charge for the same equipment.

Same-day delivery in the Twin Cities metro for orders before 3 PM CT. Full factory warranty on every unit.