What This Error Means

The furnace control detected an open thermal-limit, rollout-switch or reverse-airflow safety circuit. The burners are shut down because the furnace may be overheating or another safety condition is present.

Symptoms You May Notice

Burners shut off early; blower may continue running; furnace cycles on and off; reduced heat; fault returns during longer heating cycles; possible unusually hot cabinet or supply air.

Common Causes

Dirty filter; blocked return or supply airflow; closed registers; incorrect blower airflow setting; dirty evaporator coil; failing blower motor; open high-limit switch; tripped rollout switch; combustion or venting problem; damaged wiring.

Safe Troubleshooting Steps

1. Turn off power and allow the furnace to cool. 2. Replace a dirty filter with the correct size and type. 3. Open supply registers and confirm return grilles are unobstructed. 4. Inspect for obvious airflow restrictions. 5. Do not reset a rollout switch without identifying the cause. 6. A qualified technician should verify temperature rise, blower airflow, limit continuity, heat-exchanger condition, burner operation and venting.

Parts Commonly Involved

High-limit switch; rollout switches; reverse-airflow switch; blower motor; filter; evaporator coil; integrated furnace control; wiring; heat exchanger and venting.

When to Call a Technician

Yes if the code returns after correcting a dirty filter or obvious airflow blockage.

Safety Notice

Do not bypass limit or rollout switches. A rollout fault may indicate a dangerous combustion or heat-exchanger problem.

Manufacturer Documentation