The high heat temperature sensor, also called the high heat plenum switch, monitors plenum temperature and controls the blower circuit to protect the furnace from overheating.

This sensor sits in the supply plenum and closes or opens a control circuit based on temperature, so when temperatures rise above a safe threshold the switch opens to interrupt the normal control sequence. A properly functioning switch keeps the blower timed with the heating cycle by allowing the blower relay to energize only when temperatures are appropriate, and it also ensures the blower stays on to dissipate heat if the switch opens unexpectedly. If the switch is open because of an overtemperature event the control logic may force the blower to run continuously to cool the plenum and prevent heat buildup. When diagnosing intermittent blower operation first verify actual plenum temperatures with an infrared thermometer and compare them to the switch rating to confirm a thermal event is occurring. Inspect for restricted return or supply airflow by checking filter condition, duct obstructions, closed vents, and blower wheel cleanliness because poor airflow raises plenum temperature and can trip the switch. When replacing the switch match the temperature rating and terminal configuration exactly and mount it in the same location so sensing remains accurate.

A common issue is a false open caused by dirty air filters or blocked return vents which raises plenum temperature and trips the sensor, so clean or replace filters and restore airflow to prevent nuisance runs. A failed or corroded switch can fail open and force the blower to run constantly, so test continuity at normal temperatures and replace the switch if it does not close. Wiring or loose terminal connections can mimic switch failure and cause intermittent blower behavior, so tighten and clean connections and trace the control circuit with a meter before replacing components.

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