High Lockout
FAIL POINT B on Worcester Bosch Greenstar Si

Your boiler has suffered an electrical failure that has caused it to lose power and stop providing heating and hot water.

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

Fuse F2 blown. Potential transformer failure (resistance above 100 ohms) or board failure.

What causes this fault?

This fault occurs when a component on the boiler's internal circuit board fails or short-circuits, cutting off the main power supply to the unit. The most common reason is an electrical surge or moisture ingress that causes the fuse to blow or the transformer to burn out. Effectively, the boiler has lost its 'brain' and cannot safely manage the electrical components needed to fire up.

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 AView →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 →

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 →