Furnace High Limit Switch: What It Does and Why It Keeps Tripping

A high limit switch that keeps tripping is one of the most common furnace service calls in Minnesota. The furnace starts, runs briefly, then shuts down mid-cycle. The LED blinks 4 times (on Goodman furnaces). The house isn't getting warm. Understanding what the high limit switch does and why it trips helps you diagnose and fix the problem — often without a service call.

What Is the High Limit Switch?

The high limit switch is a thermal safety device mounted in the furnace's supply air plenum — the sheet metal chamber above the heat exchanger where heated air collects before being distributed through supply ducts. It monitors the temperature of the air in this plenum.

If the plenum temperature exceeds the switch's rated limit (typically 160–200°F depending on model), the switch opens, cutting power to the gas valve. The furnace stops heating. The blower continues running to cool the heat exchanger down. Once temperatures drop sufficiently, the switch resets automatically and the furnace attempts to restart.

The high limit switch protects against heat exchanger overheating, which can cause permanent damage or cracking. It's doing exactly what it's designed to do when it trips — the question is why temperatures got high enough to trip it.

Why the High Limit Switch Trips

Dirty or Clogged Air Filter (Most Common — 70%+ of Cases)

A clogged filter restricts the airflow across the heat exchanger. Less air passing over the heat exchanger means less heat is removed — temperatures rise until the limit trips. This is by far the most common cause of high limit faults.

Fix: Replace the filter. The limit switch will auto-reset once the heat exchanger cools. Restart the furnace with the new filter. If it completes cycles normally, you've solved it. Going forward, change filters every 1–3 months — more frequently in dusty homes or with high-MERV filters. See our filter replacement guide.

Blocked or Closed Supply Registers

Closed registers create the same effect as a dirty filter — increased static pressure, reduced airflow, rising heat exchanger temperature. Open all supply registers throughout the home. Check that furniture or rugs aren't blocking any registers. See our airflow guide.

Blocked Return Air

A single blocked or undersized return air intake starves the blower of air. Check the return air grille (usually the largest grille in the home — hallway, stairwell, or utility room) for blockage. Furniture positioned against the return grille dramatically reduces system airflow.

Failing Blower Motor

If the blower motor is running slowly or intermittently due to failing bearings or a weak capacitor, it moves less air than designed — insufficient to keep the heat exchanger cool. A motor that runs fine when cold but struggles after warming up is a classic sign of failing bearings. See our blower motor guide.

Blower Speed Set Too Low

After a furnace installation or repair, if the blower speed taps were set incorrectly (too slow for the duct system), the furnace may overheat even with clean filters and open registers. A technician can verify and adjust blower speed taps on PSC motors, or reprogram ECM motor settings.

Oversized Furnace

A furnace significantly oversized for the home fires at full capacity, heats the heat exchanger faster than the duct system can remove heat, and trips the limit. This is a design/installation issue. Solutions include reducing firing rate (some furnaces allow this), increasing duct capacity, or in extreme cases, replacing with a correctly sized unit.

When the Limit Switch Itself Has Failed

High limit switches can fail in the open position — meaning they trip and don't reset even after the heat exchanger cools. If you've addressed all the airflow causes above and the limit still doesn't reset after a 10–15 minute cooldown, the switch itself may be bad.

High limit switches cost $15–$50 and are relatively easy to replace. Identify the switch (cylindrical disc mounted on the plenum, with two wires), disconnect power, swap the switch, reconnect, and test. Use the same rated temperature as the original.

Don't Bypass the High Limit Switch

Some homeowners or inexperienced technicians bypass a tripping high limit switch to "fix" the problem. This is dangerous — the switch is protecting the heat exchanger from damage and preventing a potential fire. Bypassing it removes that protection. Diagnose and fix the root cause instead.

If your furnace is frequently tripping the high limit and is 15+ years old with other wear signs, it may be time to consider replacement. Browse Goodman furnaces at factory-direct pricing: furnace.direct/collections/heating.

Related reading: Filter Replacement Guide | Goodman Diagnostic LED Codes | Blower Motor Guide