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.
20 documented codes
6 most common
1 DIY-safe checks
5 engineer needed

The 6 most common faults

01
A1 High DIY-safe

Your boiler has detected that there is not enough water circulating inside the system to keep it running safely.

What to try first

Check the pressure gauge on the front of the boiler; if it is below 1 bar, your system needs to be re-pressurised.Locate the external filling loop connected to your boiler and open the valves to allow water in until the gauge reads 1.5 bar.Close the filling loop valves tightly and reset the boiler.

Est. cost
£80-150
Parts
Central heating pump, Pressure relief valve, Expansion vessel
Full guide for A1
02
d1 High Engineer

Your boiler has lost communication with a key internal component that tells it how to regulate your heating and hot water, causing the system to stop working as a precaution.

Est. cost
£150-300
Parts
Low Voltage Switch Module, Main Control Board, Wiring Harness
Full guide for d1
03
FC High Engineer

Your boiler's electronic display panel is failing to communicate with the internal computer, which prevents the unit from operating normally.

Est. cost
£150-300
Parts
User interface (display) module, Wiring harness, Main printed circuit board (PCB)
Full guide for FC
04
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
05
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
06
A5 High Engineer

Your boiler is having trouble reading the temperature of the internal hot water store, so it has stopped producing hot water to prevent an error.

Est. cost
£150-250
Parts
Hot water NTC sensor, Wiring harness
Full guide for A5
Parts most often replaced

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

Central heating pumpPressure relief valveExpansion vesselLow Voltage Switch ModuleMain Control BoardWiring Harness

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 20-code list for the Worcester Bosch Greenstar 25i (ZWB 7-25)

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →