High Lockout
d1 on Worcester Bosch Greenstar Ri

Your boiler is struggling to monitor the temperature of the water returning from your radiators, which may cause the system to stop working reliably.

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

Return sensor fault.

What causes this fault?

This fault occurs when the internal sensor responsible for measuring the temperature of the water coming back into the boiler has malfunctioned or lost its connection. This is usually caused by the sensor's internal components wearing out over time or from moisture damaging the electrical pins on the wiring harness. When the boiler cannot accurately measure this temperature, it shuts down to prevent the system from 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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