Worcester Bosch Greenstar 42CDi Fault codes & diagnostics

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

Combi Natural GasLPG 42 kW 2006-2012 Discontinued

Replaced by: Greenstar 42CDi Classic

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 8 High 1 Medium 2 Low
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 12 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
Fast flash

Your boiler is still sensing a flame even after it has been told to turn the gas off, which is a safety conflict.

Emergency Engineer only £150-350

Parts commonly replaced

  • Gas Valve
  • Ionisation Probe
  • Printed Circuit Board (PCB)
Full explanation for Fast flash
No flash

Your boiler is undergoing a standard restart process because the reset button was held down, and it will return to normal operation shortly.

Low DIY-safe

What to check first

  1. Wait for 30-60 seconds for the boiler to complete its internal startup checks.
  2. Ensure any 'Reset' light has stopped flashing.
  3. Check if the display returns to showing the current temperature or 'P' status.
Full explanation for No flash
Slow flash

Your boiler thinks it can see a flame still burning even though it has told the system to shut down.

High Engineer only £180-450

Parts commonly replaced

  • Gas Valve
  • Ionisation Electrode
  • Control Board (PCB)
Full explanation for Slow flash
Digital codes
-Ii-

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

Low DIY-safe

What to check first

  1. Wait 15 minutes for the cycle to complete automatically
  2. Ensure your thermostat is calling for heat during this time
  3. Do nothing else; the boiler will return to normal operation on its own
Full explanation for -Ii-