High Lockout
A5 on Worcester Bosch Greenstar 25HE

Your boiler's temperature sensor responsible for monitoring stored hot water has developed a fault, preventing it from reading the temperature correctly.

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

Heat store NTC sensor 2 defective

What causes this fault?

This fault occurs when the electrical resistance in the sensor used to monitor your hot water storage reaches an impossible value, suggesting the component has failed internally. It is usually caused by natural wear and tear of the sensor's delicate internal elements or, occasionally, a loose connection in the wiring harness that links 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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