BoilerFaultCodesThe 6 most common faults
Your boiler has stopped working because it has failed to light or has detected a dangerous build-up of heat.
Why it happens: This fault occurs when the boiler tries to light but fails to detect a flame, or if the internal temperature becomes unsafely high. It is often caused by a lack of fuel reaching the burner, a worn-out ignition spark, or a pump failure that prevents water from moving, leading to immediate overheating.
Check that your gas supply is turned on (e.g., check other gas appliances)Check the pressure gauge is between 1.0 and 1.5 barPress and hold the reset button for five seconds
Your boiler has detected an internal communication or airflow issue that has caused it to shut down for safety.
Why it happens: This fault occurs when the boiler's internal computer loses communication with its key safety components or senses that air isn't moving through the system correctly. It often signifies that the fan cannot spin properly or the sensors responsible for monitoring heat and airflow have detected an unsafe operating condition. These sensors act as a safety net, shutting the system down to prevent damage or the release of exhaust gases into the home.
Full guide for Flash 4 times per secondYour boiler has detected an issue with the sensor that manages your hot water, meaning you will likely only have heating or very inconsistent water temperatures.
Why it happens: This fault occurs when the internal sensor responsible for measuring the temperature of your hot water fails or sends an incorrect signal to the main control board. The most common reason is that the sensor has physically degraded over time or developed a wiring fault, preventing the boiler from knowing how hot the water actually is. As a safety precaution, the boiler will often stop providing hot water to prevent accidental scalding.
Full guide for LED 4 flash/secondYour boiler is failing to light the flame, which means it has stopped working to keep you safe.
Full guide for lock-outYour boiler has stopped working because it failed to successfully light its internal flame.
Full guide for LOCKOUTYour boiler has stopped working because one of its internal electrical safety fuses has blown, preventing the system from powering up.
Full guide for F1on the Worcester Bosch 24si II
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