High Lockout
A5 on Worcester Bosch Greenstar CDi Compact

Your boiler's temperature sensor responsible for monitoring the hot water storage is faulty or has lost connection, meaning the system cannot safely heat your 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

Tank (Heat Bank) NTC sensor defective.

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring the temperature of your stored hot water fails to send a clear signal to the main control board. The most common reason is that the sensor has developed an internal electrical fault or its wiring has become loose or corroded over time. Because the boiler no longer knows how hot the water is, it shuts down as a safety precaution to prevent overheating.

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 →E5View →E9View →

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