BoilerFaultCodesThe 6 most common faults
Your boiler is failing to ignite the burner at the required power, meaning it cannot provide heating or hot water.
Why it happens: This fault occurs when the boiler's control system attempts to fire the burner at full power but fails to detect the necessary electrical signal or gas pressure. It is typically caused by a failing gas valve that cannot open fully or a fault within the internal control box that prevents power from reaching the ignition components. In some cases, a loose or damaged wiring connection between these two parts is the root cause.
Full guide for POINT DYour boiler is struggling to switch between providing heating and hot water, meaning you likely have no hot water available.
Why it happens: This fault occurs when the boiler's internal brain tries to switch the water flow from your radiators to your hot taps but receives no response from the switching mechanism. The most common reason is that the small electric motor responsible for moving the valve has burnt out or the internal valve itself has become seized due to a buildup of debris and sludge within the system.
Full guide for POINT IYour boiler has stopped because the water pressure inside your heating system has dropped too low to function safely.
Locate the internal filling loop (a silver flexible hose with one or two valves attached under the boiler).Slowly open the valve(s) to allow mains water into the system until the pressure gauge reads between 1.0 and 1.5 bar.Close the valve(s) tightly and restart the boiler.
Your boiler has stopped working because the water pressure inside the heating system has dropped too low to operate safely.
Locate the external filling loop, which is a silver braided hose connected between two pipes under your boiler.Open the valves at both ends of the filling loop to allow cold mains water to enter the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone, typically between 1.0 and 1.5 bar.Close both valves firmly and ensure the hose is tight to prevent any further pressure loss.
Your boiler has lost the signal from the sensor that monitors your hot water temperature, which means it cannot accurately heat your water.
Full guide for A7Your boiler has detected that the internal pump, which pushes heat around your home, is not spinning correctly and the system has shut down to prevent damage.
Full guide for A8on the Worcester Bosch Highflow 440 CDi
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