BoilerFaultCodes.com

Worcester Bosch Greenstar 12i System Fault Codes

System Natural GasLPG 12 kW 2005-2015 Discontinued
GC Number: 41-311-73

Replaced by: Greenstar i System

This boiler uses indicator light combinations

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

Most common fault codes

A8
Your boiler's internal computer has lost contact with the main control panel, preventing it from starting up correctly.
Gas Safe engineer required · £180-450
AC
Your boiler's main control board has lost communication with an internal plug-in module or sensor, meaning it cannot currently operate.
Gas Safe engineer required · £180-450
b1
Your boiler's main internal computer cannot find its unique identity chip, causing the system to stop working for safety.
Gas Safe engineer required · £120-£350

All 30 fault codes

Code Severity Plain English
2 pulses Low Your boiler has been manually set to run at its lowest power setting for servicing and just needs to be switched back to its normal operating mode.
5 pulses Low Your boiler has been manually put into a high-power service mode by a person, and it simply needs to be switched back to normal operation.
A8 High Your boiler's internal computer has lost contact with the main control panel, preventing it from starting up correctly.
AC High Your boiler's main control board has lost communication with an internal plug-in module or sensor, meaning it cannot currently operate.
b1 High Your boiler's main internal computer cannot find its unique identity chip, causing the system to stop working for safety.
Blue light: 2 pulses, Lockout reset button: No light Low Your boiler is not broken; it has simply been put into a manual testing mode and needs to be switched back to its normal setting.
Blue light: 5 pulses, Lockout reset button: No light Low Your boiler is not broken; it has simply been put into a temporary testing mode used by engineers.
Blue light: Fast flash, Lockout reset button: Flashing High Your boiler has encountered a computer failure and has gone into a safety shutdown mode, meaning you will have no heating or hot water until it is repaired.
Blue light: Fast flash, Lockout reset button: Off High Your boiler has gone into a safety lockdown because it cannot confirm that internal components like the fan or temperature sensors are working correctly.
Blue light: Off, Lockout reset button: Off High Your boiler appears to have no power at all, meaning the screen and lights are completely dark and it will not respond.
Blue light: On, Lockout reset button: Off High Your boiler is receiving power but failing to start when you ask for heating or hot water, likely due to a faulty external control or internal communication issue.
Blue light: Slow flash (normally off, flashes on), Lockout reset button: Flashing (reset required) High Your boiler has failed to light a flame, which means it has entered a lockout mode and stopped providing heating and hot water as a safety precaution.
Blue light: Slow flash (normally on, flashes off), Lockout reset button: Flashing (reset required) High Your boiler has switched itself off because it has detected that it is getting too hot, likely due to a circulation issue or a blockage.
C1 High Your boiler's internal fan isn't spinning fast enough to safely clear away exhaust fumes, causing the system to shut down for your protection.
C4 High Your boiler has detected that the internal fan is spinning too slowly to safely clear waste gases, so it has shut down as a precaution.
C6 High Your boiler's internal fan is spinning faster than it should, which has caused the system to shut down for safety.
E2 High Your boiler has detected a fault with the internal sensor that monitors the temperature of the water heading to your radiators, causing the system to shut down for safety.
E9 High Your boiler has sensed that it is getting dangerously hot and has shut itself down to prevent damage or overheating.
EA High Your boiler has tried to light several times but has been unable to detect a flame, causing the system to shut down for safety.
F0 High Your boiler's internal computer has encountered a software or electronic error and can no longer control the heating properly.
F7 High Your boiler thinks it can still see a flame inside even though it has been told to turn off, which is a safety conflict that prevents it from starting up again.
FA Emergency Your boiler is still detecting a flame even though it has tried to turn the gas supply off, which is a safety concern that requires an engineer.
Fast flash High Your boiler's internal computer has suffered a critical failure and can no longer control the heating system.
Fast flashing reset button High Your boiler has encountered a general error and needs to be restarted to see if the system can clear the fault itself.
Fd Low Your boiler is showing this code simply because the reset button was held down for too long or pressed by mistake, which has confused the internal software.
Flue blockage High Your boiler has detected that waste gases cannot safely leave the property through the flue pipe, causing the system to shut down for safety.
Flue overheat Emergency Your boiler has shut down because it detected that the exhaust gases leaving the system have reached a dangerously high temperature.
P1, P2, P3, P1... Low Your boiler is simply going through its starting sequence and checking its systems before beginning to work.
Slow flashing reset button Low Your boiler is working normally, but it is providing a friendly reminder that its annual safety and maintenance service is now due.
Volatile lockout High Your boiler has stopped running and entered a safety shutdown mode, which is indicated by the reset button flashing.

Status & diagnostic codes

These codes indicate your boiler's operating state or a diagnostic reading — not necessarily a fault. Status codes (S.) mean normal operation; diagnostic codes (D.) are informational readings used by engineers.

Code Type Description
d3 Diagnostic Your boiler has lost its connection to an essential safety internal plug, meaning it currently cannot communicate correctly with its control system.

Worcester Bosch Greenstar 12i System is discontinued — time for a new one?

Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.

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