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 lost power or experienced an electrical error while it was already trying to show a different fault code.
Why it happens: This fault occurs when the boiler loses electrical power at the exact moment it was trying to record or display a different technical error. It is usually caused by a loose connection in the internal wiring or a failing main circuit board that can no longer process electrical signals correctly. Essentially, the boiler's internal computer has crashed while trying to report a problem, leaving it in a confused state.
Locate the reset button on the front control panelPress and hold the reset button for five secondsWait for the boiler to restart and see if a different fault code appears
Your boiler has shut down because the water inside it has become dangerously hot.
Why it happens: This fault occurs when the water inside the boiler cannot move away from the burner fast enough, causing it to reach a dangerously high temperature. The most common reasons are a failed internal pump that isn't pushing the water around, or a blockage caused by a build-up of sludge and debris in the system. When this heat spike is detected, the boiler shuts down immediately as a safety precaution to prevent internal damage.
Check if your radiator valves are fully open to allow water to circulate.Ensure your system pressure is between 1 and 1.5 bar on the front gauge.Restart the boiler by holding the 'reset' button for five seconds once the system has cooled down.
Your boiler is failing to detect a flame when it tries to start, which means it will not provide any heating or hot water until the issue is resolved.
Why it happens: This fault occurs when the boiler's internal sensor fails to detect a flame, even though it may have tried to spark and light several times. The most common reason is that the gas is not reaching the burner properly or the sensor responsible for 'seeing' the flame has become dirty or worn out.
Check if your gas supply is active by testing another gas appliance like a hob.Check if your gas meter has credit (if using a pre-payment meter).Gently press the Reset button (the blue light or a 'reset' button on the fascia) for five seconds.
Your boiler has lost its flame suddenly while it was running, causing it to stop heating your home for safety.
Why it happens: This fault occurs when the boiler’s control system suddenly loses the signal that confirms a flame is present while it is firing. The most common reason is that the flame is physically blowing out due to a gas supply issue, or the internal sensing probe has become too dirty or worn to detect the heat properly. It can also be caused by the boiler 'choking' on its own exhaust gases if there is a problem with the flue or air supply.
Check if other gas appliances like a hob are working to ensure you have a gas supplyCheck that your condensate pipe (outside) is not frozen during cold weatherPress the Reset button for five seconds and see if the boiler restarts
on 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