Defro
Fan Overrun / Stuck Relay
Boiler fan runs continuously even when there is no heating or hot water demand.
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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.
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Possible Causes
Defro PCB fan relay stuck closed, Faulty flue gas temperature sensor reading high, Control logic error on Defro main board, External control wiring backfeeding signal
How to Fix / Troubleshooting
Safety: Isolate power before inspecting the PCB.
- Check demand: Confirm that no external controls (thermostats, timers) are calling for heat and that the boiler is not in frost protection mode.
- Sensor reading: View diagnostic menu (if available) to check flue gas temperature reading. If it is unrealistically high, see E13.
- Power reset: Turn off power for several minutes, then restore. If the fan immediately starts with no demand, the PCB relay may be stuck.
- PCB replacement: A technician should confirm and replace the Defro main control board if the fan output is permanently energized.
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Repair Difficulty
Hard
4/5
Required Part
Defro main control board
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