E21
Keston Internal Communication / PCB Sensor Bus Fault

Boiler shows E21, display may indicate sensor mismatch or internal communication error

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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 main PCB, Loose internal harness connections, Damaged sensor bus wiring, Electrical interference

How to Fix / Troubleshooting

Safety: Isolate power before accessing internal electronics. Avoid static discharge to PCB components.

User checks:

  • Attempt a single reset. If E21 returns, professional service is required.

Engineer checks:

  • Inspect all low-voltage harness connections between sensors and the Keston main PCB; reseat and secure connectors.
  • Check for signs of water ingress or corrosion on the PCB and connectors.
  • Verify correct earthing and that no high-voltage cables are routed alongside sensor wiring causing interference.
  • If wiring and sensors test correctly, replace the main PCB following Keston procedures and reconfigure as required.
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Repair Difficulty

Hard 4/5

Required Part

Main Control PCB
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