High Lockout
A7 on Worcester Bosch Greenstar 30HE

Your boiler is struggling to accurately measure the temperature of your hot water, which usually means your taps will only run cold or slightly warm.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Hot water NTC sensor defective.

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring the temperature of your hot water fails to send a reliable signal to the main control board. The most common reason is natural wear and tear or corrosion on the sensor's probe, which prevents it from reading how hot the water is, causing the boiler to shut down the burner for safety. In some cases, it can also be triggered by a loose or damaged connection in the wiring that links the sensor to the rest of the unit.

Engineer required

This is a high severity fault. Stop using the boiler and call 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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