Medium Lockout
A4 on Worcester Bosch Highflow 300

Your boiler's hot water sensor is struggling to communicate with the main control board, meaning the system cannot accurately monitor the temperature of your hot water.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

DHW sensor fault. DHW sensor 10000 - 12000 ohms at room temperature. Wiring and connections to sensor.

What causes this fault?

This fault occurs when the sensor responsible for monitoring your hot water temperature fails to send a clear signal to the boiler's internal computer. The most common reason is that the sensor has physically degraded over time or a small leak has caused corrosion on the electrical connections. Without this data, the boiler stops heating water as a safety precaution to prevent it from becoming too hot.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

A7View →E9View →

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