High Lockout
A7 on Worcester Bosch ZWB 7-25

Your boiler is struggling to accurately measure the temperature of the water it is heating, which will likely prevent you from getting hot water through your taps.

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 is defective.

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring hot water temperature starts sending incorrect readings or stops working entirely. The most common reason is that the sensor has suffered a terminal electrical failure or has become heavily corroded after years of constant heat cycling.

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 →

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