High Lockout
05 on Vokera Syntesi 25

Your boiler is having trouble 'talking' to your wall-mounted thermostat or remote control unit.

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

Appliance is connected to the optional remote control (RC05) and there is a communication problem.

What causes this fault?

This fault occurs when the digital handshake between the boiler's internal computer and the wall-mounted controller is broken. The most common reason is a loose or damaged connection in the low-voltage wiring that links the two components, though it can also be caused by an internal electronic failure within the remote control unit itself.

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

01View →07View →

Related guides you might find useful

Boiler won't turn on — 9-step checklist

Work through these before the call-out.

Read →

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →