Worcester Bosch Greenstar 42CDi Fault codes & diagnostics

14 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural GasLPG 42 kW 2006-2012 Discontinued
GC Numbers 47-406-1047-406-11

Replaced by: Greenstar 42CDi Classic

Download the Worcester Bosch Greenstar 42CDi manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 12 High 1 Medium
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-406-10 Greenstar 42 CDi 2006–2012
47-406-11 Greenstar 42 CDi 2006–2012
Just want the most common Worcester Bosch Greenstar 42CDi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 14 documented codes

Tap any card for details

LED / flash-pattern codes
2 flashes

Your boiler has detected that there isn't enough water circulating through the system, which is preventing it from heating up safely.

High DIY-safe £150-£350

What to check first

  1. Check the pressure gauge on the front of the boiler; if it is below 1 bar, it needs topping up.
  2. Locate the internal or external filling loop and carefully add water until the gauge reads between 1 and 1.5 bar.
  3. Bleed any trapped air from your radiators using a radiator key.
  4. Reset the boiler to clear the fault code.

Parts commonly replaced

  • Circulating Pump
  • Expansion Vessel
  • Automatic Air Vent
Full explanation for 2 flashes
3 flashes

Your boiler is having trouble measuring the temperature of your hot water, which usually means your taps will only provide cold or lukewarm water.

Medium Engineer only £90-160

Parts commonly replaced

  • Hot water NTC thermistor
  • Connecting wiring harness
Full explanation for 3 flashes
4 flashes

Your boiler's internal computer chip is missing or has lost its connection, meaning the system cannot identify which model it is and has stopped working for safety.

High Engineer only £120-£350

Parts commonly replaced

  • HCM Code Plug
  • Printed Circuit Board (PCB)
Full explanation for 4 flashes
5 flashes

Your boiler has stopped working because the fan that helps remove waste gases is not spinning at the correct speed.

High Engineer only £280-450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness
Full explanation for 5 flashes
6 flashes

Your boiler's temperature sensor has failed, meaning the system can no longer monitor how hot the water is and has shut down as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Central Heating Flow NTC Sensor
  • Connecting Wiring Harness
Full explanation for 6 flashes
7 flashes

Your boiler has shut down because it has detected that the water inside is getting too hot, triggering a safety cut-out to prevent damage.

High DIY-safe £120-280

What to check first

  1. Check that your radiator valves are open and not turned to zero.
  2. Check the boiler pressure gauge and top up to 1.5 bar if it is low.
  3. Press the reset button to see if the fault clears once the system has cooled down.

Parts commonly replaced

  • Overheat Thermostat
  • Central Heating Temperature Sensor
  • Circulation Pump
Full explanation for 7 flashes
8 flashes

Your boiler is failing to light the gas, which means your heating and hot water will not work until the issue is resolved.

High DIY-safe £100-£250

What to check first

  1. Check if other gas appliances in your home, like a hob, are working to confirm your gas supply is on
  2. Ensure your prepaid gas meter has enough credit
  3. Press the reset button on the boiler for five seconds

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Electrode
  • Gas Valve
  • Spark Lead
Full explanation for 8 flashes
9 flashes

Your boiler’s internal computer has encountered a critical electronic failure and can no longer control the heating and hot water.

High Engineer only £300-£550

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Control Box
Full explanation for 9 flashes
Digital codes
A7

Your boiler is struggling to accurately measure the temperature of the water used for your taps and shower, which will likely prevent hot water from working correctly.

High Engineer only £120-200

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC sensor

Related codes

Full explanation for A7
b1

Your boiler's computer is having trouble recognising the internal memory chip that tells it how to operate correctly.

High Engineer only £150-300

Parts commonly replaced

  • Code plug
  • Printed Circuit Board (PCB)

Related codes

Full explanation for b1
C6

Your boiler's internal air blower is struggling to spin at the correct speed, which prevents the unit from safely venting exhaust gases and forces it to shut down.

High Engineer only £250-450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Flue sensor

Related codes

Full explanation for C6
E2

Your boiler has stopped working because it cannot accurately detect the temperature of the water used for your radiators.

High Engineer only £150-250

Parts commonly replaced

  • Flow NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for E2
E9

Your boiler has detected that it is getting too hot and has automatically shut down to prevent any damage or safety issues.

High Engineer only £150-300

Parts commonly replaced

  • NTC Sensor
  • Primary Heat Exchanger
  • Pump
  • Limit Thermostat

Related codes

Full explanation for E9
F7

Your boiler has detected a flame signal when it shouldn't be running, which is a safety lockout preventing the unit from operating.

Emergency Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Flame detection electrode
  • Gas valve

Related codes

Full explanation for F7
Status & diagnostic codes

1 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

-Ii- Status Your boiler is performing a routine automatic safety check to fill its internal drainage system following a power cut or restart.