High Lockout
A7 on Worcester Bosch CDi Classic

Your boiler's hot water sensor has failed or has a broken connection, meaning you likely won't have any hot water.

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

The hot water negative thermistor coefficient (NTC) sensor is defective – the hot water sensor or connecting leads need checking.

What causes this fault?

This fault occurs when the boiler's internal computer loses communication with the sensor responsible for monitoring hot water temperature. The most common reason is that the sensor has physically failed due to age or internal wear, though it can also be triggered by loose or corroded electrical wiring connecting the sensor to the main control board.

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 →E9View →

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