E12
Main
Control PCB Communication or Internal Fault
Boiler shows E12, no response to thermostat demand, display may be blank or frozen
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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
Failed main PCB, Corrupted firmware, Loose internal connectors, Voltage spikes or poor earthing
How to Fix / Troubleshooting
Safety: Isolate power before removing the boiler case. Electronic work should be done by a qualified engineer.
- Power cycle: Turn off the boiler at the fused spur for 2–3 minutes, then restore power and see if E12 clears.
- Check supply: Verify that the boiler has a stable 230V supply and that the polarity and earth are correct.
- Inspect external controls: Disconnect external controls (room thermostat, programmer) at the boiler terminal strip and fit a link across the call-for-heat terminals to rule out control wiring faults.
- Engineer tasks:
- Inspect the Main PCB for signs of burning, bulging capacitors, or water damage.
- Check all low-voltage harnesses and connectors between the display board and main PCB.
- Replace the Main control PCB and reconfigure parameters according to the boiler model.
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Repair Difficulty
Hard
4/5
Required Part
Main Control PCB
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