BoilerFaultCodesThe 6 most common faults
Your boiler is getting too hot too quickly and is shutting itself down to prevent damage.
Why it happens: This fault occurs when water cannot travel through the boiler fast enough to carry heat away, causing the internal temperature to spike dangerously high. The most common reasons are a failed circulation pump that isn't pushing the water or a significant blockage within the pipework or heat exchanger. In some cases, it may simply be that the internal sensors have become faulty and are providing inaccurate heat readings to the control board.
Check that your radiator valves are fully openEnsure the central heating pump is runningCheck if the system water pressure is between 1.0 and 1.5 bar
Your boiler has failed to ignite or stay lit, meaning your heating and hot water have stopped working.
Why it happens: This fault occurs when the boiler's control system cannot confirm that a flame has been successfully established after several attempts. The most common reasons are a worn-out component that senses the flame, a lack of gas reaching the burner, or a build-up of carbon deposits on the ignition parts.
Check if your gas supply is active by testing another gas appliance like a hob.Try resetting the boiler using the 'R' or reset button on the control panel.Ensure your system pressure is between 1.0 and 1.5 bar on the pressure gauge.
Your boiler has shut itself down as a safety precaution because it has detected an overheating issue within the system.
Why it happens: This fault occurs when the water inside the boiler reaches an unsafe temperature, triggering a safety sensor to cut the power to the burner. The most common reasons include a failing pump that isn't moving water away from the burner quickly enough, or a buildup of debris and sludge that is restricting flow through the heat exchanger.
Full guide for 02Your boiler has detected a sequence of ignition or flame detection errors and has shut itself down for safety.
Why it happens: This fault occurs when the boiler's internal computer fails to detect a reliable flame during the ignition process. The most common reasons are worn-out sparks that can no longer light the gas, or a dirty sensor that cannot 'see' the flame even after it has started. It can also be caused by a lack of gas reaching the burner due to a faulty valve or a blockage in the air supply.
Full guide for 04Your boiler has detected a sequence of ignition or flame detection errors and has shut itself down for safety.
Why it happens: This fault occurs when the boiler's internal computer fails to detect a reliable flame during the ignition process. The most common reasons are worn-out sparks that can no longer light the gas, or a dirty sensor that cannot 'see' the flame even after it has started. It can also be caused by a lack of gas reaching the burner due to a faulty valve or a blockage in the air supply.
Full guide for 05Your boiler has detected a sequence of ignition or flame detection errors and has shut itself down for safety.
Why it happens: This fault occurs when the boiler's internal computer fails to detect a reliable flame during the ignition process. The most common reasons are worn-out sparks that can no longer light the gas, or a dirty sensor that cannot 'see' the flame even after it has started. It can also be caused by a lack of gas reaching the burner due to a faulty valve or a blockage in the air supply.
Full guide for 06on the Viessmann Vitodens 333-F
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 Viessmann 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