Worcester Bosch Greenstar Fault codes & diagnostics

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

Regular Gas 2004-present
Choose your boiler type

The Greenstar range comes in 5 types

Pick the one you have. The fault codes, GC numbers and manual differ by type.

30 18 codes · 1 GC number 12 Ri 5 codes · 2 GC numbers 24 Ri 5 codes · 2 GC numbers 18Ri 5 codes · 2 GC numbers FS 30 CDi Regular (2008-2015) 13 codes · 1 GC number
Each model has its own manual A family spans several manuals, one per model or generation. Pick your model above and its page has the manual its fault codes were read from.

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 3 High 2 Low
Just want the most common Worcester Bosch Greenstar faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 6 documented codes

Tap any card for details

LED / flash-pattern codes
Fast flash

Your boiler has detected a critical internal error that caused it to shut down for safety reasons and will not restart on its own.

High Engineer only £150-350 Shared across the Greenstar range

Parts commonly replaced

  • Fan
  • Printed Circuit Board (PCB)
  • Flue gas sensor
Full explanation for Fast flash
Slow flash (normally off, flashes on)

Your boiler has tried to light several times but failed to ignite, so it has safely shut itself down for your protection.

High DIY-safe £100-£250 Shared across the Greenstar range

What to check first

  1. Check that your gas supply is turned on and other gas appliances like the hob are working.
  2. Check your prepayment gas meter if you have one to ensure you haven't run out of credit.
  3. Locate the reset button on your boiler and press it firmly for about 5 seconds to attempt a restart.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for Slow flash (normally off, flashes on)
Slow flash (normally on, flashes off)

Your boiler has detected that the flue is getting too hot and has automatically shut down to keep your home safe.

Emergency Engineer only £150-300 Shared across the Greenstar range

Parts commonly replaced

  • Flue thermostat
  • High limit thermostat
  • PCB

Related codes

Full explanation for Slow flash (normally on, flashes off)
Digital codes
2 pulses

Your boiler has been left in a manual testing mode after a service or inspection and needs to be switched back to its normal operating setting.

Low DIY-safe £0-80 Shared across the Greenstar range

What to check first

  1. Locate the service mode switch (or dial) on the boiler control panel.
  2. Ensure the switch is moved from the 'Min' or 'Service' position back to the 'Normal' or 'Off' position.
  3. Perform a simple reset by pressing the reset button to clear the flashing light.
Full explanation for 2 pulses
5 pulses

Your boiler has been left in its internal testing mode, which is usually only used by engineers during a service or setup.

Low DIY-safe £0-80 Shared across the Greenstar range

What to check first

  1. Locate the horizontal dial on the front of the boiler control panel.
  2. Check if the dial is turned all the way to the right (maximum position).
  3. Turn the dial back to your desired temperature setting to exit service mode.

Parts commonly replaced

  • None
Full explanation for 5 pulses
N/A

Your boiler requires a specific installation or commissioning step to be completed before it will unlock and allow normal operation.

High Engineer only £80-150 Shared across the Greenstar range
Full explanation for N/A
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