F3 displayed, burner does not start or shuts down, fan may run briefly
Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
Possible Causes
How to Fix / Troubleshooting
Safety first: Turn off power and gas. Allow the appliance and flue to cool completely. Risk of burns and flue gas exposure.
- Check flue system: Inspect the flue outlet and air intake for obstructions (ice, bird nests, debris). Ensure the concentric flue of the Protherm heater is correctly assembled and sealed.
- Inspect flue gas sensor: Locate the flue gas NTC sensor on the flue hood or outlet. Check for damaged insulation, discoloration, or loose mounting.
- Test sensor resistance: Disconnect the sensor and measure its resistance with a multimeter. Compare with Protherm temperature/resistance chart. Replace if out of range or open/shorted.
- Check wiring to PCB: Ensure the sensor harness is intact and firmly connected to the main control board.
- Reset and test: After correcting issues, reset the fault by cycling power and run the heater while monitoring flue temperature and operation.
Warning: Never operate the heater with a known flue blockage or bypass the flue gas sensor; this is extremely dangerous.
Repair Difficulty
Required Part
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