BoilerFaultCodesThe 6 most common faults
Your boiler is failing to detect a flame and has safely shut down to prevent unburned gas from flowing.
Why it happens: This fault occurs when the boiler's control system cannot confirm that a flame has successfully lit, leading it to shut down for safety. The most common reasons include a lack of gas reaching the burner, worn-out ignition components that fail to spark, or a dirty internal sensor that can no longer 'see' the flame.
Check if you have credit on your gas meter or if other gas appliances in the house are working.Inspect the white plastic condensate pipe outside for signs of freezing and thaw it with warm water if necessary.Press the 'Reset' button on the boiler fascia for five seconds to see if the system restarts.
Your boiler has stopped working because the internal fan is not spinning fast enough to safely clear away exhaust fumes.
Why it happens: This fault occurs when the boiler's control system detects that the fan is not spinning at the correct speed to safely expel waste gases through the flue. The most common reasons include a buildup of dust and debris within the fan motor, a loose electrical connection, or a failing control board that is unable to send the correct power signal to the fan.
Full guide for C1Your boiler has detected that its internal fan is spinning at the wrong speed, which means it cannot safely exhaust fumes and has stopped working as a result.
Why it happens: This fault occurs when the boiler's control system loses the ability to regulate the fan speed effectively, often due to a mechanical failure or an electrical communication error. The most common reason is a failing fan motor or a fault within the main circuit board that prevents it from sending the correct signals to manage airflow.
Full guide for C6Your boiler has detected a problem with the internal sensor that monitors the temperature of the water heading to your radiators, causing the system to shut down for safety.
Why it happens: This fault occurs when the electrical connection to the temperature sensor on the main heating pipe becomes unstable or fails completely. The most common reason is that the sensor itself has developed an internal electrical fault or the wiring has become loose or damaged over time, preventing the boiler from safely monitoring heat levels.
Full guide for E2Your boiler thinks there is still a flame burning even though it has commanded the gas to turn off.
Why it happens: This fault occurs when the boiler's internal sensor detects a flame signal at a time when the gas should be completely shut off. The most common reason is a sensor that has become covered in carbon deposits or moisture, though it can also be caused by a gas valve that is failing to close fully or a fault within the main control board.
Full guide for F7Your boiler’s internal computer and main controls have lost their connection, preventing the system from starting up.
Why it happens: This fault occurs when the 'brain' of your boiler can no longer talk to the control panel or the internal sensors. It is usually caused by moisture affecting the electrical circuits, a loose connection in the internal wiring, or a critical failure of the main printed circuit board itself.
Full guide for A8on the Worcester Bosch Greenstar 25 HE Combi
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