Common problems guide

The most common Worcester Bosch Greenstar 42CDi problems

The faults most likely to send a Worcester Bosch Greenstar 42CDi 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.
12 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
E9 High DIY-safe

Your boiler has shut down because it has detected that the water inside is Getting too hot, which is a safety feature to prevent damage.

What to try first

Check if any radiator valves are closed and open them fullyEnsure your system pressure is between 1 and 1.5 bar on the gaugePress the 'Reset' button for five seconds to restart the boiler

Est. cost
£120-350
Parts
Overheat Thermostat, Circulation Pump, Main Heat Exchanger (Flushing)
Full guide for E9
02
EA High DIY-safe

Your boiler is failing to light the gas or isn't detecting that the flame has started, causing the system to shut down for safety.

What to try first

Check if other gas appliances in your home, like a hob, are working to confirm your gas supply is active.If it is freezing outside, check that your external condensate pipe hasn't frozen and blocked.Press and hold the reset button for five seconds to see if the boiler restarts.

Est. cost
£100-£350
Parts
Ignition Electrode, Flame Sensor (Ionisation Probe), Gas Valve
Full guide for EA
03
A1 High Engineer

Your boiler is struggling because the internal water pump is stuck or cannot circulate water properly, preventing your heating and hot water from working.

Est. cost
£250-450
Parts
Circulation pump, Pump head
Full guide for A1
04
b1 High Engineer

Your boiler's computer memory chip is loose or missing, meaning the system doesn't know which model it is and has stopped working for safety.

Est. cost
£90-180
Parts
Code plug, Printed Circuit Board (PCB)
Full guide for b1
05
C6 High Engineer

Your boiler’s internal fan is not spinning fast enough to safely clear away exhaust fumes, which has caused the system to shut down for safety.

Est. cost
£250-£450
Parts
Fan Assembly, Main PCB, Air Pressure Switch
Full guide for C6
06
E2 High Engineer

Your boiler has detected a faulty internal temperature sensor, which means it cannot accurately monitor the water temperature and has shut down as a safety precaution.

Est. cost
£100-£180
Parts
CH Flow NTC Sensor, Connecting Wiring Harness
Full guide for E2
Parts most often replaced

on the Worcester Bosch Greenstar 42CDi

Overheat ThermostatCirculation PumpMain Heat Exchanger (Flushing)Ignition ElectrodeFlame Sensor (Ionisation Probe)Gas Valve

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 12-code list for the Worcester Bosch Greenstar 42CDi

Related guides

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