High Lockout
9U on Worcester Bosch Highflow 440 CDi

Your boiler has stopped working because the internal fan is not spinning fast enough to safely clear waste gases from the system.

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

Fan speed too low. Fan harness and connections.

What causes this fault?

This fault occurs when the boiler's control system detects that the fan is not rotating at the required speed to safely expel exhaust gases. The most common reasons are a build-up of debris within the fan's bearings, a loose connection in the wiring, or a failing motor that can no longer reach its target velocity.

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

C1View →C6View →A1View →

Related guides you might find useful

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →