Medium Lockout
A7 on Worcester Bosch Greenstar 25 HE Combi

Your boiler is struggling to accurately measure the temperature of your hot water, which usually prevents it from heating the water correctly.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

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 NTC sensor defective (short or open circuit)

What causes this fault?

This fault occurs when the sensor responsible for monitoring your hot water temperature fails to send a clear signal back to the main control board. The most common reason is that the sensor has developed a physical breakdown or electrical short after years of constant temperature changes, though loose or corroded wiring can also mimic this failure. Because the boiler no longer knows how hot the water is, it stops heating it as a safety precaution to prevent scalding.

Engineer required

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

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Related fault codes

A8View →B1View →E9View →

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