BoilerFaultCodesThe 6 most common faults
Your boiler has sensed that it is getting too hot and has automatically shut down for safety.
Why it happens: This fault occurs when the water inside the boiler reaches an unsafe temperature, triggering a safety switch to kill the power. The most common reasons are a pump failure that stops water from moving around the system or a build-up of debris causing a blockage in the heat exchanger.
Check that your system pressure is between 1 and 1.5 bar on the front gaugeTop up the water pressure using the white internal or external filling loop if it is too lowEnsure all radiator valves are fully open and there are no blockagesPress the 'reset' button on the boiler control panel once pressure is corrected
Your boiler has stopped working because it cannot light a flame, most likely due to an interruption in the gas supply or a part that has worn out.
Why it happens: This fault occurs when the boiler tries to start up but fails to detect a steady flame after several attempts. The most common reasons are a temporary interruption in your gas supply, a build-up of carbon on the internal sensors, or a component failure like a worn-out ignition lead.
Check if other gas appliances, like a hob, are working to confirm your gas supply is onIf you have a prepaid meter, ensure you have sufficient creditPress and hold the 'Reset' button for five seconds to see if the boiler restartsIn freezing weather, check if your white plastic condensate pipe outside has frozen and gently thaw it with warm water
Your boiler has detected that there is not enough water circulating through the system, which could be due to a leak or a mechanical issue with the pump.
Why it happens: This fault occurs when the boiler detects a serious lack of water pressure or circulation, often caused by a leak within the system or a failing internal pump. If the pump is spinning without enough water (running dry) or has become seized with debris, the boiler will shut down to prevent the components from overheating or burning out. In some cases, a faulty air vent can also cause an air lock that tricks the sensors into thinking the system is empty.
Full guide for A1Your boiler's hot water sensor has failed or has a broken connection, meaning you likely won't have any hot water.
Why it happens: This fault occurs when the boiler's internal computer loses communication with the sensor responsible for monitoring hot water temperature. The most common reason is that the sensor has physically failed due to age or internal wear, though it can also be triggered by loose or corroded electrical wiring connecting the sensor to the main control board.
Full guide for A7Your boiler has lost its connection to the external heating controls, meaning it cannot receive instructions on when to turn on or off.
Why it happens: This fault occurs when the boiler's internal computer loses its digital handshake with your room thermostat or programmer. The most common reason is a hardware failure in the wireless receiver plugged into the boiler front, or degraded electrical wiring that physically links the controls to the main circuit board.
Full guide for A8Your 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.
Why it happens: This fault occurs when the boiler's control board loses communication with a small, removable chip called the code plug. It is usually caused by the plug vibrating loose over time or failing due to a hardware fault on the main circuit board. Without this chip, the boiler cannot verify which model it is or what safety settings to use, so it shuts down immediately.
Full guide for B1on the Worcester Bosch CDi Classic
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