Worcester Bosch Greenstar 25 HE Combi Fault codes & diagnostics

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

Combi Natural GasLPG 25 kW 2005-2007 Discontinued

Replaced by: Greenstar 25i

Download the Worcester Bosch Greenstar 25 HE Combi manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 14 High 2 Medium 2 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 19 documented codes

Tap any card for details

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
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
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
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
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
d3

Your boiler has lost the connection to its internal control bridge, which is preventing it from starting up properly.

High Engineer only £120-250

Parts commonly replaced

  • Control board
  • Wiring harness
  • Link wire

Related codes

Full explanation for d3
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
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
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
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
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

8 operating states & engineer diagnostics

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

0.0 Diagnostic Your boiler is currently reporting that a previous fault has occurred and needs to be manually cleared or reset to resume normal operation. 2.5 Diagnostic 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. 2.6 Diagnostic 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. 5.0 Diagnostic 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. 6.8 Diagnostic Your boiler is confused by how long it is taking to heat your hot water and is stuck in a constant cycle. 6.A Diagnostic 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. ECO-button lit Status 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. ECO-button not lit Status 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.