BoilerFaultCodesThe 6 most common faults
01
6.4 High DIY-safe
Your boiler has stopped working because the water pressure inside the heating system has dropped too low to operate safely.
What to try first
Locate the external filling loop, which is a flexible braided hose usually connected to the pipework underneath the boiler.Open the small valves at either end of the loop to allow mains water into the system.Monitor the pressure gauge on the front of the boiler until the needle sits in the green zone, typically between 1 and 1.5 bar.Close both valves tightly and remove or disconnect the filling loop if required.
02
E9 High Engineer
Your boiler has stopped working because it cannot detect the temperature of the water flowing through the system, likely due to a loose wire or a faulty sensor connection.
Full guide for E903
A1 High Engineer
Your boiler is failing to light the internal flame needed to heat your water, so it has safely shut itself down to prevent gas from building up.
Full guide for A104
A5 High Engineer
Your boiler's hot water storage tank is experiencing a sensor error, meaning it currently cannot accurately detect the temperature of the water inside.
Full guide for A505
A7 High Engineer
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 A706
A8 High Engineer
Your 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
Filling loopPressure relief valvePressure sensor (transducer)NTC temperature sensorWiring harnessPrinted Circuit Board (PCB)
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