High Lockout
FAIL POINT A on Worcester Bosch Greenstar Si

Your boiler has lost power because it is switched off at the front, there is no electricity reaching the unit, or an internal fuse has blown.

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

Facia on/off switch turned off, or no 230V AC live supply across Terminal ST12 pins L and N, or fuse F1 blown.

What causes this fault?

This fault occurs when the boiler's internal computer loses its electrical connection, essentially leaving the unit without the power it needs to operate. The most common reasons are that the main power switch on the front panel has been turned off, a fuse on the circuit board has popped, or there is an external issue with the electrical supply to the house. It can also happen if the control board itself has suffered a component failure and can no longer process electricity.

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

FAIL POINT BView →FAIL POINT CView →

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 →