BoilerFaultCodesThe 6 most common faults
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 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's internal fan isn't spinning fast enough to safely clear away exhaust fumes, causing the system to shut down for your protection.
Why it happens: This fault occurs when the boiler's control system detects that the internal fan is spinning slower than the required safety threshold. The most common reason is a buildup of dirt or mechanical wear within the fan motor itself, though it can also be caused by loose electrical connections or a failing control board failing to send the correct signal.
Full guide for C1Your boiler has detected that the internal fan is spinning too slowly to safely clear waste gases, so it has shut down as a precaution.
Why it happens: This fault occurs when the boiler's control system detects that the fan is not reaching the required speed to safely expel exhaust fumes. The most common reasons are a buildup of dirt inside the fan housing, electrical components on the fan failing, or a fault within the main circuit board that prevents it from sending the correct power to the motor.
Full guide for C4Your boiler's internal fan is spinning faster than it should, which has caused the system to shut down for safety.
Why it happens: This fault occurs when the boiler's control system loses the ability to regulate the fan, causing it to spin at an unsafe speed. The most common reason is an electrical failure within the fan's internal motor or a malfunctioning circuit board that is sending incorrect voltage to the component.
Full guide for C6on 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
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
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