BoilerFaultCodesThe 6 most common faults
Your boiler has paused its initial setup process because the automatic air-venting sequence has not finished successfully.
Why it happens: This fault occurs during the commissioning stage when the boiler's internal computer determines that air is still trapped within the heat exchanger or primary circuit. It is often triggered because the automatic venting program was interrupted or the system's air bleed valve is stuck, preventing the unit from safely transitioning into normal operation. No gas will be ignited until the boiler is satisfied that all air has been purged to prevent overheating.
Check that the manual air vent on top of the boiler is openEnsure the central heating system pressure is between 1.0 and 1.5 barPress and hold the 'Mode' or 'Reset' button to restart the commissioning sequence
Your boiler is continuing to detect a flame even after it has been told to shut down, which is a safety precaution to prevent gas issues.
Why it happens: This fault occurs when the boiler's sensors detect heat or a flame signal even after the gas supply should have been cut off. The most common reason is a gas valve that is failing to close fully or a faulty electrode that is sending a false 'flame present' signal to the main control board.
Full guide for EFYour boiler is having trouble 'talking' to its internal pump, which means it cannot flow water around the system to heat your home.
Why it happens: This fault occurs when the boiler's main control board loses its digital connection with the internal circulation pump. The most common reason is a hardware failure within the pump’s electronic motor head or a loose communication cable, meaning the boiler can no longer control the speed of the water flow. Without this connection, the system shuts down as a safety precaution to prevent the heat exchanger from overheating.
Full guide for A9Your boiler is currently stuck in an incomplete setup mode, meaning the installation process wasn't fully finished or the system hasn't been properly coded.
Why it happens: This fault occurs when the boiler's internal computer has not been told exactly what type of heating system it is connected to. It is usually triggered if a technician replaced a part or installed the unit but didn't complete the final software configuration steps. Until this digital 'handshake' is finished, the boiler stays in a safe standby mode and refuses to fire up.
Full guide for b1Your boiler has not been fully configured for operation or there is a communication problem between the internal controls and the burner assembly.
Why it happens: This fault occurs when the boiler's internal computer cannot successfully communicate with the burner system or hasn't been properly programmed to start. It is often caused by a loose connection, a faulty ignition lead, or a breakdown in the electronic control board that manages the gas flow. Effectively, the boiler has safety-locked because it cannot verify that it is safe to ignite the flame.
Full guide for b4Your boiler has failed to complete its initial setup process, meaning it hasn't properly cleared air from the system or finished its first startup sequence.
Why it happens: This fault occurs when the boiler's automatic commissioning program fails to complete, often because the system hasn't been fully bled of air during setup. It signifies that the internal sensors cannot confirm a stable flame or safe operating conditions, causing the computer to stop the sequence for safety. The root cause is usually a snag during the initial 'purging' phase or a component like the ignition electrode failing to spark correctly.
Full guide for b5on the Viessmann Vitodens
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