Furnace locks out with overheat indication, supply temperature extremely high, burner stops

Safety: Turn off power and allow the boiler to cool completely before touching any pipework or sensors.

• Check system pressure and valves: Verify the system pressure on the Viessmann gauge (typically 1.0–1.5 bar when cold). Ensure all isolation valves on the flow and return are fully open.
• Bleed air from radiators and boiler: Use a radiator key to bleed air from the highest radiators. On some Viessmann models, bleed the automatic air vent near the pump or heat exchanger by loosening the cap slightly.
• Verify pump operation: With power restored, set the thermostat to call for heat and listen for the internal Viessmann circulation pump. If it is silent or extremely hot to the touch, it may be seized. Check the pump connector on the control board. A seized pump requires replacement by a technician.
• Inspect high-limit sensor: Locate the safety temperature limiter (STB) on the flow pipe or heat exchanger. Check wiring for damage or loose plugs. Some models have a manual reset button on the STB; press it once if present.
• Reset the boiler: After restoring circulation and bleeding air, reset the fault. If F3 recurs, do not continue to reset; call a professional to check flow rates and sensor calibration.