BoilerFaultCodes
The most common Viessmann 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 detected an internal electronic error and has safely shut itself down to prevent further issues.
Why it happens: This fault occurs when the boiler's main control board loses its digital connection with the room thermostat. The most common reason is a loose or damaged connection wire, though it can also be caused by a software glitch or a hardware failure within the thermostat itself.
Check that your thermostat is powered on and has fresh batteries if applicable.Restart the boiler using the power switch or reset button to refresh the connection.Ensure the thermostat wiring is securely plugged into its base if it is a wall-mounted unit.
Your boiler has stopped working because there is not enough water flowing through the system to keep it running safely.
Why it happens: This fault occurs when the boiler's internal sensors detect that water isn't moving through the unit fast enough to safely carry heat away from the burner. The most common reasons are a failing internal pump that can no longer push the water or a significant blockage within the main heat exchanger. It can also be triggered by a build-up of sludge in the system pipes that restricts overall flow.
Check the pressure gauge on the front of the boiler; if it is below 1.0 bar, top it up using the filling loop.Ensure all radiator valves are fully open to allow water to circulate.Gently bleed air from your radiators using a radiator key.
Your boiler has switched itself off because it has reached a dangerously high temperature, likely due to a lack of water or poor circulation.
Why it happens: This fault occurs when the boiler's internal components detect that the heat being produced isn't being carried away quickly enough, causing the system to overheat. The most common reasons are a failed internal pump that isn't moving the water, or a significant blockage within the heat exchanger that is restricting flow. It can also be triggered by a large pocket of trapped air or a dangerously low water level in the system.
Check the pressure gauge and top up the water if it is below 1.5 barBleed all radiators to remove trapped air from the systemEnsure all radiator valves and isolation valves are fully openReset the boiler after checking water levels
Your boiler has shut down because it has detected it is getting too hot, which is usually caused by a lack of water or a circulation problem.
Why it happens: This fault occurs when the boiler's internal sensors detect that the heat exchanger has reached a dangerously high temperature, triggering a safety shutdown to prevent damage. The most common reasons are a failed internal pump that isn't moving water away from the burner quickly enough, or a significant air lock trap in the primary circuit. It can also be caused by a build-up of sludge blocking the flow of water through the narrow passages of the boiler.
Check the pressure gauge on your boiler and top up the water if it is below 1.5 barEnsure all your radiator valves are open and not switched offOnce the boiler has cooled down, press the 'Reset' button to see if the fault clears
Your boiler has failed to light the flame, which means it cannot provide any heating or hot water until the issue is resolved.
Why it happens: This fault occurs when the boiler's control system attempts to start the burner but cannot detect a flame after several attempts. The most common reasons are a lack of gas reaching the unit, worn-out spark electrodes that fail to ignite the gas, or a faulty sensor that cannot 'see' the flame even if it has successfully lit.
Check if other gas appliances in your home, like a hob, are working to confirm your gas supply is active.Ensure your gas meter has credit if you use a prepay meter.Press the Reset button on the boiler control panel once to see if the fault clears.
Your boiler's internal computer has crashed due to an ignition failure or electrical interference, preventing it from starting the burner.
Why it happens: This fault occurs when the boiler's internal computer experiences a critical communication error or fails to verify the ignition process. The most common reasons are worn-out spark electrodes that can no longer light the gas, or electrical 'noise' from nearby appliances interfering with the boiler's sensitive electronics.
Press the reset button or turn the boiler off and back on at the fused spur switchEnsure large electronic devices or powerful wireless routers are not sitting directly next to the boilerCheck if other gas appliances in your home are working to ensure gas is reaching the property