High Lockout
65:0 on Viessmann Vitodens 300-W

Your boiler is having trouble managing how it sends heat to your radiators or hot water cylinder because it cannot detect or communicate with a key internal control valve.

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

Without diverter valve

What causes this fault?

This fault occurs when the boiler's main computer loses its electronic connection with the motor that moves the internal water valve. The most common reason is that the motor unit has failed internally, though it can also be caused by loose wiring or a communication error on the main circuit board.

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

65:1View →65:2View →F9View →

Related guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on combis.

Read →

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 →