Worcester Bosch Greenstar 18 i System Fault codes & diagnostics

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

System Natural Gas 2009-2015 Discontinued
GC Number 41-311-86
Download the Worcester Bosch Greenstar 18 i System manualThe official installation and service manual (PDF), the document these fault codes were read from. 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 →

Fault codes

All 2 documented codes

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Fast flash (Flashing)

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

High Engineer only £200-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
Full explanation for Fast flash (Flashing)
Fast flash (Off)

Your boiler has stopped working because the internal fan, which is essential for safely venting gases, has failed to start up.

High Engineer only £200-350

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness
Full explanation for Fast flash (Off)
Status & diagnostic codes

8 operating states & engineer diagnostics

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

2 pulses (No light) Status Your boiler is operating normally and awaiting a command from your heating controls, meaning it is currently in standby mode. 5 pulses (No light) Status Your boiler is operating normally and there is no fault currently present, as the five-pulse indicator simply confirms the unit is performing its standard internal safety check. SI1 Diagnostic Your boiler has stopped working because an internal electrical safety fuse has blown, cutting off power to the system's control board. SI2 Diagnostic Your boiler has stopped working because an internal safety fuse has blown, likely due to an electrical surge or a faulty component inside the unit. SI3 Diagnostic Your boiler has stopped working because an internal electrical fuse has blown, cutting off the power needed for it to operate. ST6 Diagnostic Your boiler has detected a potential issue with the external frost protection sensor that prevents pipes from freezing during cold weather. ST10 Diagnostic Your boiler is struggling to detect a signal from your wall-mounted thermostat or the power supply, meaning it doesn't know when to switch on for your heating or hot water. ST19 Diagnostic Your boiler is having a communication breakdown with its internal controls or a connected thermostat, meaning it cannot receive instructions to heat your home.