E09
Defro
Communication Fault Between User Interface and Main PCB
Boiler shows E09, no response to thermostat or control signals, sometimes intermittent.
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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
Loose communication cable between Defro display panel and main PCB, Faulty Defro control panel (HMI), Faulty main control board on Defro boiler, Electrical interference or water ingress on low-voltage harness
How to Fix / Troubleshooting
Safety: Isolate power before removing the front panel or control covers.
- Inspect display cable: Remove the front cover and locate the ribbon or data cable connecting the Defro user interface to the main PCB. Ensure both ends are fully seated.
- Check for moisture: Look for signs of condensation or leaks dripping onto the control panel or PCB. If moisture is present, identify and fix the leak (e.g., from automatic air vent or pipe joint) before drying the electronics.
- Power cycle: After reseating cables, leave the boiler powered off for 2–3 minutes, then restore power and check if E09 clears.
- Professional diagnosis: If the fault persists, a technician should test the low-voltage communication bus and may need to replace either the Defro display module or the main PCB.
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Repair Difficulty
Hard
4/5
Required Part
Defro main control board
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