High Lockout
F10 on Intergas Rapid 25

Your boiler has detected a fault with the sensor that monitors water returning from your radiators, preventing the system from heating up safely.

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 short circuit

What causes this fault?

This fault occurs when the sensor responsible for measuring the temperature of the water returning to the boiler develops an electrical shortcut. The most common reason is that the internal component of the sensor has failed or the protective insulation on the connecting wires has worn away, causing a constant, incorrect electrical signal.

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 fault codes

F11View →F12View →F15View →

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 →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →