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 is providing central heating but is failing to detect when you turn on a hot tap, meaning you have no hot water.
Why it happens: This fault occurs when the boiler's internal sensors fail to detect water moving through the system when you open a tap. The most common reason is that the plastic turbine inside the flow sensor has become stuck with limescale or debris, preventing it from sending a signal to the boiler to fire up for hot water. In some cases, the temperature sensor has simply worn out and is providing an incorrect reading that tricks the system into staying off.
Full guide for No DHW function (CH OK)Your boiler is providing hot water normally, but is not receiving the signal to turn on the central heating due to a control or wiring issue.
Why it happens: This issue occurs when the boiler is ready to provide heat but is waiting for a command from your thermostat or timer that never arrives. The most common reason is a faulty room thermostat or a broken internal programmer that can no longer send an electrical signal to 'call' for heat. Because the boiler is working fine for hot water, the problem is almost always located in the external controls or the wiring that connects them to the main circuit board.
Ensure your wall thermostat is turned up higher than the current room temperature.Check that your heating programmer or timer is set to 'ON' or 'AUTO'.Replace the batteries in your wireless room thermostat if it has them.
Your boiler has been left in a service mode used for testing, meaning it is currently stuck running at a constant power level instead of operating normally.
Why it happens: This fault occurs when a manual switch inside the boiler has been left in a testing position, usually after a service or repair. It physically prevents the boiler from adjusting its flame size automatically, meaning the system cannot regulate the heat for your taps or radiators. While it is often just a switch left in the wrong position, it can also happen if the main control board becomes faulty and misreads the signal.
Full guide for Flash 8 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/secondon 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