BoilerFaultCodesThe 6 most common faults
01
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 E902
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 A103
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 A504
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 A705
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 A806
b1 High Engineer
Your boiler's computer memory chip is loose or missing, meaning the system doesn't know which model it is and has stopped working for safety.
Full guide for b1on the Worcester Bosch Highflow 550 CDi
NTC temperature sensorWiring harnessPrinted Circuit Board (PCB)Ignition electrodeIonisation probeGas valve
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