Boiler displays E4 and will not operate.

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.

Possible Causes

Failed DHW NTC sensor, Loose DHW sensor connection, Scale build-up around DHW sensor pocket, Faulty PCB DHW sensor circuit.

Safety first: Isolate the electrical power and allow the boiler to cool.

Check if fault is DHW-only: Test central heating and hot water separately. If only DHW is affected, this supports a DHW sensor issue.
Visual check of DHW sensor: On combi models, locate the DHW NTC on the plate heat exchanger or DHW outlet pipe. Ensure the sensor is firmly clipped in and the connector is secure.
Descale DHW side (engineer): Heavy limescale can insulate the sensor. An engineer may need to chemically descale the plate heat exchanger and inspect the sensor pocket.
Engineer tasks: Measure DHW NTC resistance, compare to specifications, and replace if out of range. Verify wiring continuity to the PCB.

Hard 4/5

DHW Temperature Sensor (NTC)

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