High Lockout
Fault 12 / 170 on Worcester Bosch Greenstar CDi Compact

Your boiler's internal computer has lost communication with a central safety component or an external controller, causing the system to shut down for safety.

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

System configuration: BUS device missing. HLC has tripped or is faulty.

What causes this fault?

This fault occurs when the boiler's main control board loses a stable electrical connection with the safety sensors that monitor for overheating. It is usually caused by a loose wiring connection or a sensor that has permanently failed due to wear and tear. Because the boiler cannot verify that the water temperature is safe, it shuts down immediately to prevent damage.

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

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