E40
Baxi
External Control / Bus Communication Error
Boiler displays E40, often associated with external controls or communication faults
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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
Faulty Baxi wired room thermostat or timer, Incorrect wiring on control terminals, Damaged low-voltage bus cable, PCB communication port fault
How to Fix / Troubleshooting
Safety first: Isolate power before touching any wiring.
User checks:
- If you have a wireless thermostat, replace the batteries and ensure the receiver is powered and paired.
- Check that any programmer or timer is set to call for heat and not in OFF or holiday mode.
- Power cycle both the boiler and the control (if possible) to reset communication.
Professional actions:
- An engineer or electrician should verify the control wiring, test the thermostat, and check the boiler’s low-voltage terminals.
- If the control is faulty, replace with a compatible Baxi control or approved equivalent.
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Repair Difficulty
Medium
3/5
Required Part
Room Thermostat / Control Interface
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