Boiler shows F3 and will not ignite, fan may not run

Safety: Turn off power and gas. Working on flue and combustion components must be done by a qualified technician due to risk of carbon monoxide.

Inspection steps:

• Access the combustion chamber and locate the flue gas temperature sensor mounted on or near the flue outlet or in the flue hood.
• Check that the sensor is firmly fixed and not physically damaged or sooted up. Clean light deposits carefully without damaging the sensor.
• Inspect the wiring harness from the sensor to the PCB for heat damage, brittleness, or broken insulation. Repair or replace as needed.
• Measure the sensor resistance at room temperature and compare with the Protherm flue NTC specification. Replace the sensor if readings are out of range.
• Check the flue system for obstructions, incorrect termination, or collapsed sections. A blocked flue can cause excessive flue temperatures and repeated F3 errors. Any flue work must comply with local regulations.
• If the sensor and flue are OK, the PCB may be misreading the sensor. A technician should test and replace the PCB if necessary.

After repair: Reassemble the sealed chamber correctly, ensuring all gaskets are intact. Restore power and gas, reset the boiler, and verify stable operation without F3.