BoilerFaultCodesThe 6 most common faults
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 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 06Your 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 08Your boiler has detected a sequence error or a flame signal where there shouldn't be one, causing the burner to shut down completely for safety.
Why it happens: This fault occurs when the boiler detects a persistent flame signal even after the burner should have switched off, or if the internal electrical checks fail during start-up. The most common reason is a build-up of carbon or moisture on the ionisation probe, which tricks the system into thinking a fire is still burning. In some cases, a fault within the main control board or damaged ignition wiring prevents the boiler from safely managing the combustion process.
Full guide for 07Your boiler has shut down because it is struggling to discharge exhaust fumes safely or is detecting a significant lack of air flow.
Why it happens: This fault occurs when the boiler's internal sensors detect that exhaust gases cannot exit the building or fresh air cannot enter the system properly. It is usually caused by a heavy buildup of debris within the heat exchanger or a blockage in the flue pipe, which prevents the burner from firing safely. In some cases, a blocked condensate trap can also cause water to back up, effectively choking the combustion process.
Full guide for 1AYour boiler has sensed that the internal pump is malfunctioning or water is not circulating correctly, which has caused the system to stop as a safety precaution.
Why it happens: This fault occurs when the boiler's internal sensors detect that water is not moving through the heat exchanger as quickly or efficiently as it should. The most common reason is a mechanical failure of the circulation pump or a buildup of debris that has physically jammed the pump's internal components. In some cases, it may also be triggered by a communication error where the main control board is unable to tell the pump to spin.
Full guide for A2on 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
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
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