F4 displayed, furnace locked out, fan may continue running

Safety first: Turn off power and allow the furnace to cool completely. Do not bypass any safety thermostat.

• Step 1 – Check airflow: Inspect and replace dirty air filters. Ensure all supply and return grilles are open and not blocked by furniture or debris.
• Step 2 – Inspect heat exchanger and fan: Remove the burner/heat exchanger access panel. Look for heavy dust buildup on the heat exchanger fins and blower wheel. Clean carefully with a soft brush and vacuum.
• Step 3 – Check circulation pump (if hydronic Protherm unit): For Protherm furnaces that heat water, verify that the system pump is running and that system valves are open. A seized pump will cause rapid overheating.
• Step 4 – Reset safety thermostat: Locate the manual‑reset safety thermostat (often on the heat exchanger outlet). If it has a reset button, press it once after the unit has cooled. If it trips repeatedly, do not keep resetting—find the cause.
• Step 5 – Sensor testing: If airflow and water circulation are normal, the STB sensor or NTC temperature sensor may be faulty. A technician should measure sensor resistance and replace the defective sensor.