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Discover the most common boiler fault codes.
If you’ve found a fault code flashing on your display, browse our list below and see the common cause.
If you’ve found a fault code flashing on your display, browse our list below and see the common cause.









Boiler Fault Codes
Worcester Bosch Greenstar i Series (e.g., 25i, 30i)
Meaning: No flame detected – likely ignition failure.
Possible Cause: Gas supply issue, electrode problem, or blocked flue.
Meaning: Flame signal lost during operation.
Possible Cause: Weak flame, flue fault, or gas valve issue.
Meaning: Low system pressure.
Possible Cause: Pressure below 1.0 bar – check for leaks or refill system.
Meaning: Fan not running or running too slowly.
Possible Cause: Faulty fan or PCB issue.
Meaning: Pump fault or airlock.
Possible Cause: Seized pump, air in the system, or circulation issue.
Worcester Bosch Greenstar Si Compact
Meaning: Loss of flame signal.
Possible Cause: Faulty ignition electrode, flame sensor, or low gas pressure.
Meaning: Fan running too slowly.
Possible Cause: Fan malfunction or blocked flue.
Meaning: Flow temperature sensor fault.
Possible Cause: Sensor defective or wiring problem.
Meaning: Issue with hot water temperature sensor.
Possible Cause: Sensor fault or scale build-up in heat exchanger.
Worcester Bosch Greenstar CDi Classic (e.g., 34CDi, 38CDi)
Meaning: Ignition failure.
Possible Cause: No gas, faulty ignition leads or electrodes.
Meaning: Pump overrun or airlock.
Possible Cause: Air in system or pump fault.
Meaning: Fan not running.
Possible Cause: Faulty fan or PCB.
Meaning: Overheat detected.
Possible Cause: Pump failure, blocked system, or heat exchanger fault.
Worcester Bosch Greenstar 4000
Meaning: Boiler has failed to ignite.
Possible Cause: Gas supply problem or ignition fault.
Meaning: Fan is blocked or faulty.
Possible Cause: Debris in fan or flue, or faulty motor.
Meaning: Circulation error.
Possible Cause: Pump jammed, airlock, or blockage in system.
Worcester Bosch Greenstar Highflow
Meaning: Overheat condition.
Possible Cause: Pump failure or restricted flow.
Meaning: No flame detected.
Possible Cause: Gas valve fault or ignition issue.
Meaning: Fan error.
Possible Cause: Fan is slow or not spinning.

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