High Lockout
1025 on Worcester Bosch Greenstar 8000 Life/Style

Your boiler's internal thermometer that measures the water returning from your radiators has developed a fault and can no longer read 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

Return temp. sensor is faulty

What causes this fault?

This fault occurs when the sensor responsible for measuring the water coming back from your radiators fails to send a reliable signal to the boiler's control board. The most common reason is that the internal electrical components of the sensor have worn out or corroded over time, or a wire has become loose or damaged. Because the boiler can no longer verify the water temperature, 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.

Find a Gas Safe engineer
Search the official register →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Radiators cold at the bottom

Sludge build-up — needs flushing, not bleeding.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →