BoilerFaultCodesThe 6 most common faults
Your boiler has detected that waste gases cannot safely leave the property through the flue pipe, causing the system to shut down for safety.
Why it happens: This fault occurs when the boiler's internal sensors detect that the exhaust gases are not being cleared from the system effectively. The most common reasons include a physical obstruction in the external flue pipe, such as a bird's nest or debris, or a mechanical failure of the fan that pushes these gases out. Because these gases are dangerous, the boiler will immediately stop working as a safety precaution.
Check the external flue pipe for any visible obstructions like bird nests, debris, or leavesIn freezing weather, check the end of the flue for ice buildup and carefully clear itPress and hold the blue reset button for 3 to 5 seconds to see if the boiler restarts
Your boiler has stopped running and entered a safety shutdown mode, which is indicated by the reset button flashing.
Why it happens: This fault occurs when the boiler's control system detects a failure in the ignition process or a loss of flame signal during operation. The most common reasons are that the burner has failed to light correctly or the internal sensors can no longer verify that a flame is present for safety. This results in the boiler shutting down immediately to prevent unburnt gas from building up in the system.
Press and hold the flashing Reset button for five seconds.Wait for the blue light to stop flashing and stay on constant.Ensure your gas supply is active and your system pressure is between 1 and 1.5 bar.
Your boiler has encountered a general error and needs to be restarted to see if the system can clear the fault itself.
Why it happens: This fault occurs when the boiler's internal control system detects an inconsistency or failure during the ignition or operating cycle. The most common reasons include a failure to strike a spark, a signal interruption between the main control board and the internal components, or a fan that is not reaching the required speed. It essentially acts as a safety 'stop' to prevent the boiler from operating under uncertain conditions.
Locate the reset button on the front control panel.Press and hold the reset button for exactly five seconds.Wait for the light to stop flashing and see if the boiler ignites.
Your boiler has tried to light several times but has been unable to detect a flame, causing the system to shut down for safety.
Why it happens: This fault occurs when the boiler's internal computer tries to start the burner multiple times but cannot confirm that a steady flame has been established. The most common reasons include a lack of gas supply to the unit, a blocked pipe that removes waste water, or dirty internal sensors that can no longer 'see' the flame effectively.
Check if your gas supply is active by testing other gas appliances like a hobEnsure your gas meter has credit if you use a pre-payment meterPress and hold the 'Reset' button for five seconds to see if the boiler restartsCheck that the condensate pipe (outside white plastic pipe) isn't frozen if the weather is very cold
Your boiler's internal computer has encountered a software or electronic error and can no longer control the heating properly.
Why it happens: This fault occurs when the boiler's main control board experiences a persistent data error or a physical failure in its electronic circuitry. It is essentially a communication breakdown between the boiler's Brain and its components, often caused by age-related degradation of the circuit board or a sudden electrical surge.
Press the 'Reset' button on the front of the boiler once.If it does not reset, turn the power supply to the boiler off at the fused spur switch.Wait 60 seconds and then turn the power back on.
Your boiler's internal computer has suffered a critical failure and can no longer control the heating system.
Why it happens: This fault occurs when the boiler's main control board suffers a hardware failure or a memory error, meaning it can no longer process data or send instructions to parts like the pump or fan. The most common reasons for this are age-related component wear or an electrical surge that has damaged the delicate circuitry inside the control box.
Full guide for Fast flashon the Worcester Bosch Greenstar 12i System
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Worcester Bosch shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue