Common problems guide

The most common Worcester Bosch Greenstar 25i (ZWB 7-25) problems

The faults most likely to send a Worcester Bosch Greenstar 25i (ZWB 7-25) into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.

The 6 most common faults

01

5.2 High Engineer

Your boiler has detected an internal communication error with its main control unit, which has caused it to stop working for safety reasons.

Est. cost: £150-350
Parts: Main printed circuit board (PCB), Wiring harness

Full guide for 5.2

02

9.3 High Engineer

Your boiler's main computer control panel is failing to process instructions correctly, which has caused it to stop working for safety reasons.

Est. cost: £250-450
Parts: Main Electronic Control Unit (PCB)

Full guide for 9.3

03

P1 High Engineer

Your boiler is struggling to power up correctly, likely because an internal electrical safety fuse has failed.

Est. cost: £150-250
Parts: 24V fuse, Main printed circuit board (PCB)

Full guide for P1

04

P2 High Engineer

Your boiler is struggling to start up because a small internal fuse responsible for powering the electronic controls has stopped working.

Est. cost: £120-220
Parts: Control board fuse, Main control board (PCB)

Full guide for P2

05

P3 High Engineer

Your boiler's electronic control system has failed to start up because a small internal protective fuse has blown.

Est. cost: £150-250
Parts: Control board fuse, Printed Circuit Board (PCB)

Full guide for P3

06

3.3 High Engineer

Your boiler is failing to detect the flame needed to heat your water because the internal sensor is dirty or worn out.

Est. cost: £120-220
Parts: Ignition electrode, Ionisation probe, Gasket kit

Full guide for 3.3

Parts most often replaced

on the Worcester Bosch Greenstar 25i (ZWB 7-25)

Main printed circuit board (PCB)Wiring harnessMain Electronic Control Unit (PCB)24V fuseControl board fuseMain control board (PCB)

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

You can smell gas or see signs of a leak
The Worcester Bosch shows an Emergency or High severity code
The boiler keeps locking out after repeated resets
You've tried the DIY checks and the fault hasn't cleared
There's visible water leaking from the boiler
The flame is yellow or orange instead of blue

See the full 14-code list for the Worcester Bosch Greenstar 25i (ZWB 7-25)