Worcester Bosch Greenstar 25CDi Fault codes & diagnostics

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

Combi Natural GasLPG 25 kW 2005-2015 Discontinued
GC Number 47-114-55

Replaced by: Greenstar 25i

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 1 Low
Just want the most common Worcester Bosch Greenstar 25CDi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 11 documented codes

Tap any card for details

LED / flash-pattern codes
2 flashes

Your boiler has lost water pressure, meaning there isn't enough water inside for the pump to circulate properly.

High DIY-safe £60-120

What to check first

  1. Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.
  2. Locate the filling loop (braided silver hose) underneath the boiler and open the valves to top up the pressure.
  3. Bleed your radiators to remove any trapped air, then re-check the pressure gauge.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Circulating pump
Full explanation for 2 flashes
3 flashes

Your boiler's hot water temperature sensor has stopped working, which means the system cannot accurately heat your water.

Medium Engineer only £90-160

Parts commonly replaced

  • Hot Water NTC Sensor
  • Connecting Lead / Wiring Harness
Full explanation for 3 flashes
4 flashes

Your boiler's internal computer brain is unable to recognize a small identifying chip that tells it how to operate safely.

High Engineer only £120-280

Parts commonly replaced

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

Your boiler has identified that the fan responsible for clearing exhaust fumes is not spinning at the correct speed, preventing it from starting safely.

High Engineer only £250-£450

Parts commonly replaced

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

Your boiler's temperature sensor is failing to communicate correctly, which prevents the system from knowing how hot the water is and causes it to shut down for safety.

High Engineer only £90-160

Parts commonly replaced

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

Your boiler has overheated because the water inside is not moving correctly or has become too hot, causing a safety sensor to shut the system down.

High DIY-safe £100-£280

What to check first

  1. Check the pressure gauge is between 1.0 and 1.5 bar
  2. Bleed all radiators to remove trapped air
  3. Ensure at least two radiator valves are fully open to allow water flow
  4. Reset the boiler using the reset button

Parts commonly replaced

  • Overheat Thermostat
  • Circulating Pump
  • Temperature Sensor (NTC)
Full explanation for 7 flashes
8 flashes

Your boiler is failing to light because it cannot detect a flame, meaning your heating and hot water will not work.

High DIY-safe £90-280

What to check first

  1. Check that your gas meter is turned on
  2. If you have a pre-payment meter, ensure you have credit
  3. Check if other gas appliances, like a cooker, are working
  4. Press the Reset button for five seconds

Parts commonly replaced

  • Ignition Electrode
  • Ionisation Probe
  • Gas Valve
  • PCB
Full explanation for 8 flashes
9 flashes

Your boiler's internal computer has encountered a critical communication error and can no longer control the heating process safely.

High Engineer only £280-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Ignition Leads
  • Programmer Interface Module
Full explanation for 9 flashes
Rapid flashes

Your boiler is sensing a flame even when it should be turned off, causing the system to lock down for safety.

High Engineer only £120-450

Parts commonly replaced

  • Ionisation Electrode
  • Flame Sensing Cable
  • Main PCB
  • Burner Seals
Full explanation for Rapid flashes
Slow flashes

Your boiler is still sensing a flame even though it has tried to shut off the gas, which is a safety concern that requires a professional inspection.

Emergency Engineer only £180-£450

Parts commonly replaced

  • Gas valve
  • Flame sensing electrode
  • Ignition leads
  • Control board (PCB)
Full explanation for Slow flashes
Digital codes
Steady blue

Your boiler is currently in a manual reset mode and simply needs you to confirm you want it to restart.

Low DIY-safe

What to check first

  1. Locate the reset button on the front control panel.
  2. Press and release the reset button once (do not hold it down for a long time).
  3. Wait a few seconds for the blue light to stop flashing or go out, and the boiler should resume normal operation.
Full explanation for Steady blue