High Lockout
Fault 12 on Worcester Bosch Greenstar CDi Compact

Your hot water tank has overheated and a safety switch has shut the system down to prevent damage.

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 cylinder temperature is so high that the cylinder High Limit Thermal Cut-out has tripped.

What causes this fault?

This fault occurs when the water inside your hot water storage cylinder reaches an unsafely high temperature, triggering a safety switch to kill the power to the boiler. The most common reason is a mechanical failure in the motorised zone valve, which gets stuck in the 'open' position and continues to heat the tank even when the thermostat tells it to stop.

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

D1View →A7View →

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