Furnace High Limit Switch Guide: Why It Trips and What to Do

Furnace High Limit Switch Guide: Why It Trips and What to Do

The high limit switch is a critical safety device that shuts off your furnace burner if the heat exchanger gets dangerously hot. Repeated high limit trips are a serious warning that something is wrong—and ignoring them can lead to heat exchanger failure or fire risk.

What the High Limit Switch Does

Mounted on the plenum or heat exchanger, the high limit switch monitors air temperature inside the furnace. If the temperature exceeds the set point (typically 160°F–200°F depending on the unit), the switch opens the control circuit and shuts off the gas burner. The blower continues running to cool the heat exchanger, then the limit switch resets when temperature drops.

Why the High Limit Switch Trips

Restricted airflow is the leading cause—typically a dirty filter blocking return air. Other causes: blocked return air vents from furniture or closed registers, dirty evaporator coil restricting summer airflow (affects winter too), failed or slow blower motor not moving enough air, oversized furnace short-cycling in a tight house.

The Danger of Repeated Trips

Every time the high limit trips, the heat exchanger experiences thermal stress from overheating. Over time this causes metal fatigue and cracking—creating a carbon monoxide risk. If your furnace trips the high limit repeatedly, solve the root cause immediately. See our heat exchanger guide and CO safety guide.

Diagnosing and Fixing High Limit Issues

Start with the filter—replace it if it's been more than 1–3 months. Check all return and supply vents are open. If the problem persists after airflow is restored, have a technician check the blower motor. See our blower motor guide. Furnace Direct stocks new Goodman systems factory-direct.