Worcester Bosch Greenstar 25 HE Combi Fault codes & diagnostics

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

Combi Natural GasLPG 25 kW 2005-2007 Discontinued
GC Number 47-114-41

Replaced by: Greenstar 25i

This boiler uses indicator light combinations

Note which lights are on, off, or flashing and at what speed. Match the combination to the cards below. Do not reset until you have noted the pattern. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 32 High 7 Medium 6 Low
Just want the most common Worcester Bosch Greenstar 25 HE Combi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 46 documented codes

Tap any card for details

LED / flash-pattern codes
Lock-out Indicator

Your boiler has shut itself down for safety because it cannot successfully light or maintain a flame.

High DIY-safe £100-350

What to check first

  1. Check if your other gas appliances (like a cooker) are working to ensure gas supply
  2. Ensure your decorative gas fire or meter valve hasn't been accidentally turned off
  3. Press and hold the reset button for five seconds once gas supply is confirmed

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve
  • PCB

Related codes

Full explanation for Lock-out Indicator
Digital codes
0.0

Your boiler is currently reporting that a previous fault has occurred and needs to be manually cleared or reset to resume normal operation.

Medium DIY-safe £80-120

What to check first

  1. Locate the reset button on your boiler control panel.
  2. Press and hold the reset button for 2 to 3 seconds.
  3. Wait a few minutes to see if the boiler ignites and the display returns to normal.

Parts commonly replaced

  • PCB (Printed Circuit Board)
  • NTC Thermistor
  • Pressure Sensor

Related codes

Full explanation for 0.0
2.4

Your boiler is temporarily pausing its burner to prevent it from switching on and off too frequently, which protects the internal parts from unnecessary wear.

Low DIY-safe

What to check first

  1. Check that all radiator valves (TRVs) are fully open to allow heat to escape the boiler
  2. Monitor the boiler for a few minutes as it should restart automatically once the timer expires
  3. Ensure the system pressure is between 1.0 and 1.5 bar on the front gauge
Full explanation for 2.4
2.5

Your boiler has shut down because the water in the heating system has become too hot, usually due to a lack of circulation or a blockage.

High DIY-safe £100-£280

What to check first

  1. Check that all radiator valves (TRVs) are turned to the highest setting
  2. Ensure the system pressure is between 1 and 1.5 bar on the gauge
  3. Bleed all radiators to remove trapped air that could be blocking water flow

Parts commonly replaced

  • NTC Thermistor (Flow temperature sensor)
  • Central heating pump
  • Main Heat Exchanger (if scaled or blocked)

Related codes

Full explanation for 2.5
2.6

Your boiler has detected that the temperature is changing too quickly or slowly between the flow and return pipes, causing it to shut down to prevent damage.

High Engineer only £120-350

Parts commonly replaced

  • Temperature sensors (NTC thermistors)
  • Circulation pump
  • Main Heat Exchanger

Related codes

Full explanation for 2.6
5.0

Your boiler is operating at its maximum heating capacity and has entered a temporary monitoring state, which is usually part of normal operation or a simple setting adjustment.

Low DIY-safe £0-120

What to check first

  1. Check that your radiator valves are fully open
  2. Ensure the system pressure is between 1 and 1.5 bar on the front gauge
  3. Turn the heating control knob down slightly and restart the boiler

Parts commonly replaced

  • No parts usually required
  • Temperature sensor (NTC thermistor)

Related codes

Full explanation for 5.0
6.8

Your boiler is confused by how long it is taking to heat your hot water and is stuck in a constant cycle.

Medium Engineer only £120-280

Parts commonly replaced

  • DHW Temperature Sensor
  • Main PCB
  • Diverter Valve

Related codes

Full explanation for 6.8
6.A

Your boiler has recorded a previous error that has triggered a lockout, meaning the unit is currently preventing itself from starting to protect the system.

High Engineer only £120-250

Parts commonly replaced

  • Ignition lead
  • Flame rectification electrode
  • Printed Circuit Board

Related codes

Full explanation for 6.A
8.d

Your boiler has detected an issue with the internal electronics that manage the burner, meaning it has safely shut down to prevent unsafe operation.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 8.d
A1

Your boiler has detected that the water inside the system is too low or has trapped air, preventing the water pump from working correctly.

High DIY-safe £90-180

What to check first

  1. Check your boiler's pressure gauge; if it is below 1 bar, increase the pressure to 1.5 bar using the filling loop.
  2. Bleed all radiators in the house to release any trapped air.
  3. Restart the boiler once the pressure is correct and the air has been removed.

Parts commonly replaced

  • System filling loop
  • Pressure relief valve
  • Central heating pump

Related codes

Full explanation for A1
Showing 11–20 of 46
A5

Your boiler is struggling to accurately measure the temperature of the water inside the storage tank, which is preventing your hot water from heating up correctly.

High Engineer only £90-160

Parts commonly replaced

  • NTC Temperature Sensor
  • Sensor Wiring Harness

Related codes

Full explanation for A5
A7

Your boiler is struggling to accurately measure the temperature of your hot water, which usually prevents it from heating the water correctly.

Medium Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC Sensor
  • Connecting wiring harness

Related codes

Full explanation for A7
A8

Your boiler’s internal computer and main controls have lost their connection, preventing the system from starting up.

High Engineer only £180-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring Harness
  • Display PCB

Related codes

Full explanation for A8
AC

Your boiler's internal computer is having trouble communicating with one of its main control parts, meaning the system cannot run.

High Engineer only £180-£450

Parts commonly replaced

  • PCB (Main Circuit Board)
  • Heatronic Module
  • Internal Wiring Harness

Related codes

Full explanation for AC
Ad

Your boiler is struggling to detect the temperature of your hot water cylinder, meaning it may not heat your water correctly until this sensor is checked or replaced.

Medium Engineer only £120-220

Parts commonly replaced

  • Cylinder temperature sensor
  • NTC thermistor harness

Related codes

Full explanation for Ad
b1

Your boiler has lost its internal identity chip and can no longer recognize its own settings or model type.

High Engineer only £120-280

Parts commonly replaced

  • Code Plug
  • Printed Circuit Board (PCB)

Related codes

Full explanation for b1
b2

Your boiler's computer is experiencing a temporary communication error which is preventing it from operating correctly.

High Engineer only £150-350

Parts commonly replaced

  • Main printed circuit board (PCB)

Related codes

Full explanation for b2
b2, b3, b4, b5,b6

Your boiler's internal computer has encountered a data corruption or communication error, preventing the system from running safely.

High Engineer only £250-£550

Parts commonly replaced

  • Main PCB
  • Code Plug

Related codes

Full explanation for b2, b3, b4, b5,b6
b3

Your boiler is struggling because it has detected an issue with the electrical board that controls its internal communications.

High Engineer only £200-400

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for b3
b4

Your boiler’s internal computer has suffered a critical memory failure and can no longer communicate with the rest of the system.

High Engineer only £250-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Code Plug

Related codes

Full explanation for b4
Showing 21–30 of 46
b5

Your boiler's internal computer has suffered a data failure and can no longer process instructions to provide heating or hot water.

High Engineer only £280-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Code Plug

Related codes

Full explanation for b5
b6

Your boiler's internal computer has suffered a permanent memory error and can no longer communicate with its own sensors.

High Engineer only £250-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Control Box Component

Related codes

Full explanation for b6
b7

Your boiler's internal computer has detected a communication failure with its ignition system, meaning it cannot safely light the burner to provide heat.

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB
  • Burner Control Unit

Related codes

Full explanation for b7
C1

Your boiler has stopped working because the internal fan is not spinning fast enough to safely clear away exhaust fumes.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Air pressure switch

Related codes

Full explanation for C1
C4

Your boiler's fan is not spinning fast enough to clear out exhaust fumes, so the system has shut down as a safety precaution.

High Engineer only £180-450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Air pressure switch

Related codes

Full explanation for C4
C6

Your boiler has detected that its internal fan is spinning at the wrong speed, which means it cannot safely exhaust fumes and has stopped working as a result.

High Engineer only £250-£450

Parts commonly replaced

  • Fan Assembly
  • Main Circuit Board (PCB)
  • Venturi

Related codes

Full explanation for C6
CA

Your boiler has detected that water is flowing through it faster than the system can manage, which may lead to inefficient heating.

Medium Engineer only £120-250

Parts commonly replaced

  • Flow sensor
  • Printed Circuit Board (PCB)
  • Pump

Related codes

Full explanation for CA
d1

Your boiler is struggling to communicate with a key internal component that monitors the heating system's water flow, preventing it from firing up reliably.

High Engineer only £150-300

Parts commonly replaced

  • LSM 5 module
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for d1
d5

Your boiler is struggling to accurately read the temperature of the water being sent out to your radiators because a sensor responsible for monitoring this flow has developed a fault.

Medium Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for d5
D6

Your boiler's internal water storage tank is overfilling, which could eventually lead to water leaking from the unit.

Medium Engineer only £120-250

Parts commonly replaced

  • Float switch
  • Inlet valve
  • Expansion vessel

Related codes

Full explanation for D6
Showing 31–40 of 46
E2

Your boiler has detected a problem with the internal sensor that monitors the temperature of the water heading to your radiators, causing the system to shut down for safety.

High Engineer only £90-160

Parts commonly replaced

  • Central Heating Flow NTC Thermistor
  • Connecting wiring harness

Related codes

Full explanation for E2
E9

Your boiler has shut down because a sensor has detected that the internal water or exhaust temperature has become dangerously high.

High DIY-safe £120-280

What to check first

  1. Check that your radiator valves are open and not turned off.
  2. Check that the boiler pressure is between 1 and 1.5 bar on the pressure gauge.
  3. Restart the boiler by holding the 'Reset' button for five seconds to see if the fault clears.

Parts commonly replaced

  • Overheat Thermostat
  • Circulating Pump
  • Heat Exchanger (Flushing)

Related codes

Full explanation for E9
EA

Your boiler is failing to detect a flame and has safely shut down to prevent unburned gas from flowing.

High DIY-safe £90-280

What to check first

  1. Check if you have credit on your gas meter or if other gas appliances in the house are working.
  2. Inspect the white plastic condensate pipe outside for signs of freezing and thaw it with warm water if necessary.
  3. Press the 'Reset' button on the boiler fascia for five seconds to see if the system restarts.

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve

Related codes

Full explanation for EA
ECO-button lit

Your boiler is currently in 'Comfort Mode', which means it keeps a small amount of water hot inside the unit so that your taps get warm faster.

Low DIY-safe

What to check first

  1. Locate the ECO button on the front control panel
  2. Press and hold the ECO button until the light turns off to deactivate Comfort mode
  3. Press and hold the button again if you wish to re-enable the feature
Full explanation for ECO-button lit
ECO-button not lit

Your boiler is currently running in its energy-saving mode, meaning it will take slightly longer to provide hot water but uses less gas to keep the system warm.

Low DIY-safe

What to check first

  1. Locate the ECO button on the front control panel
  2. Press the button once to toggle between economy and standard mode
  3. Observe if the light illuminates to confirm the boiler is back in standard operating mode
Full explanation for ECO-button not lit
F0

Your boiler's computer may have encountered a temporary glitch or a more serious internal component failure.

High DIY-safe £250-450

What to check first

  1. Locate the reset button on the front control panel.
  2. Press and hold the reset button for 3 to 5 seconds.
  3. Wait for the boiler to restart and see if the code clears.

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Control Box Unit

Related codes

Full explanation for F0
F1

Your boiler's computer is having trouble processing information, which is preventing it from starting up correctly.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for F1
F7

Your boiler thinks there is still a flame burning even though it has commanded the gas to turn off.

High Engineer only £120-350

Parts commonly replaced

  • Ionisation electrode
  • Control board (PCB)
  • Gas valve

Related codes

Full explanation for F7
FA

Your boiler thinks it still sees a flame even though the gas supply has been turned off, which is a safety conflict.

Emergency Engineer only £120-350

Parts commonly replaced

  • Gas valve
  • Ionisation electrode
  • Printed Circuit Board (PCB)

Related codes

Full explanation for FA
FC

Your boiler's electronic control panel is not communicating with the main computer system, meaning it cannot display status messages or operate correctly.

High Engineer only £150-300

Parts commonly replaced

  • User interface/display module
  • Main control board
  • Wiring harness

Related codes

Full explanation for FC
Showing 41–46 of 46
Fd

Your boiler is simply letting you know that the 'Reset' button was pressed for too long or by accident.

Low DIY-safe

What to check first

  1. Press the Reset button once more briefly to clear the code.
  2. If the code remains, switch the power to the boiler off at the fused spur switch.
  3. Wait 30 seconds and switch the power back on.

Related codes

Full explanation for Fd
P1

Your boiler is struggling to start up because an internal electrical fuse has failed, cutting power to essential components.

High Engineer only £90-160

Parts commonly replaced

  • 24V 2A Fuse
  • Printed Circuit Board (PCB)
  • Internal Wiring Harness

Related codes

Full explanation for P1
P1, P2, P3, P1...

Your boiler is currently going through a startup and self-test sequence, which usually happens after it has been switched back on or reset.

Low DIY-safe £0-120

What to check first

  1. Wait for approximately 5 to 10 minutes for the sequence to complete
  2. Ensure the boiler power switch is in the 'on' position
  3. If the code persists for over 30 minutes, try pressing the 'Reset' button once

Parts commonly replaced

  • None (Normal operation)
  • PCB (if stuck in loop)
Full explanation for P1, P2, P3, P1...
P1, P2, P3, P4

Your boiler has detected an internal computer error that is preventing it from starting up correctly.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for P1, P2, P3, P4
P2

Your boiler is failing to start up because an internal electrical fuse has blown and needs to be replaced.

High Engineer only £90-160

Parts commonly replaced

  • 24V 2A Fuse
  • Printed Circuit Board (PCB)

Related codes

Full explanation for P2
P3

Your boiler's internal electrical circuit has lost power because a small safety fuse has blown, preventing the system from starting up.

High Engineer only £90-180

Parts commonly replaced

  • 24V 2A Fast-blow fuse
  • Internal wiring loom
  • Printed Circuit Board (PCB)

Related codes

Full explanation for P3
Status & diagnostic codes

1 operating states & engineer diagnostics

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

d3 Diagnostic Your boiler has lost its connection to an external safety device or a specific internal link that tells it how to operate safely.