E20
Potterton
External Control / Thermostat Fault
Boiler shows E20 and will not start, often after wiring changes or thermostat replacement
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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
Incorrect room thermostat wiring, Open circuit on call-for-heat terminals, Faulty external controls, PCB input fault
How to Fix / Troubleshooting
Safety first: Isolate power before accessing wiring terminals.
User checks:
- Ensure the programmer is set to ON and the room thermostat is turned up above room temperature.
- Check any external controls (e.g., wireless receiver) for batteries and status lights.
Engineer checks:
- Verify correct wiring of room thermostat and programmer to the boiler's low-voltage control terminals.
- Link out the call-for-heat terminals at the boiler to test if it fires without external controls.
- Check continuity of control cables and replace if damaged.
- Replace faulty external controls or, if necessary, investigate PCB input issues.
Correct wiring and control configuration are essential for safe boiler operation.
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Repair Difficulty
Medium
3/5
Required Part
Room Thermostat / Control Wiring
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