Worcester Bosch Greenstar Si Fault codes & diagnostics

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

Combi Natural Gas 25-30 kW 1999-2002 Discontinued
GC Number 47-311-49

Replaced by: Greenstar Si Compact

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

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 →

Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-311-49 25 Si RSF 2000–2001
Just want the most common Worcester Bosch Greenstar Si faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 5 documented codes

Tap any card for details

LED / flash-pattern codes
2 lights flashing fast

Your boiler has failed to ignite properly, meaning it cannot create the flame needed to heat your home or water.

High Engineer only £120-350

Parts commonly replaced

  • Ignition Lead
  • Electrode Assembly
  • Gas Valve
  • PCB
Full explanation for 2 lights flashing fast
Fast flash

Your boiler has detected a general internal fault that requires a system reset or power cycle to clear.

High DIY-safe £80-250

What to check first

  1. Locate the main power switch for the boiler (usually a fused spur nearby) and turn it off.
  2. Wait for approximately 30 seconds.
  3. Turn the power back on and check if the flashing light stays off and the boiler fires up.

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Ignition Electrode
  • Overheat Thermostat

Related codes

Full explanation for Fast flash
Slow flash

Your boiler has detected a safety-related issue and has locked itself out to prevent further damage, requiring a manual reset to restart.

High DIY-safe £80-250

What to check first

  1. Locate the reset button on the front control panel
  2. Press and hold the reset button for at least one second
  3. Release the button and wait for the boiler to attempt to restart
  4. If the boiler refires, monitor it closely; if the light returns, call an engineer

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Slow flash
Digital codes
FAIL POINT K

Your boiler's main internal computer has developed a serious communication error and cannot operate, often because it has been exposed to water or has suffered an electrical failure.

High Engineer only £250-£450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring Harness
Full explanation for FAIL POINT K
Lock-out

Your boiler has stopped working because it cannot detect a flame, likely due to a gas supply interruption.

High DIY-safe £120-250

What to check first

  1. Check that your gas emergency control valve is in the 'on' position
  2. If you have a pre-payment meter, ensure you have credit and the meter is active
  3. Try resetting the boiler by holding the 'Reset' button for five seconds

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Condensate trap
Full explanation for Lock-out
Status & diagnostic codes

3 operating states & engineer diagnostics

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

Two red lights flash rapidly Diagnostic Your boiler's internal settings dial has been accidentally moved into an active testing mode instead of its normal operating position. F1 Diagnostic Your boiler has stopped working because an internal electrical fuse has blown, preventing the control board from powering up. F2 Diagnostic Your boiler has stopped working because an internal electrical safety fuse has blown, preventing the system from powering up.