BoilerFaultCodesThe 6 most common faults
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 detected trapped air inside, which is preventing it from starting up safely.
Why it happens: This fault occurs when air becomes trapped inside the main heat exchanger, preventing the boiler from successfully finishing its startup sequence. It is usually caused by a faulty automatic air vent or air that was not properly purged from the system after a refill or a repair.
Check that the radiator valves are open and the system is fully topped up with water to the correct pressure (usually 1.5 bar).Bleed all radiators in the house to remove any trapped air pockets.Restart the boiler to trigger the automatic venting program.
Your boiler is not lighting properly and has locked itself out for safety, which means you currently have no heating or hot water.
Why it happens: This fault occurs when the boiler tries to ignite the gas to create a flame but fails to detect one within a safe time limit. The most common reasons include a lack of gas reaching the burner, worn-out ignition components that can't create a spark, or a dirty sensor that cannot 'see' the flame even if it has lit.
Check that your gas pre-payment meter (if you have one) has credit.Ensure the gas stop tap is fully open and other gas appliances like your hob are working.Restart the boiler: Turn the control knob to the 'Reset' position until 'CL' appears on the screen, then turn it back to its original position.
Your boiler has switched itself off because it has reached a temperature that is too high, often caused by a blockage or a pump issue.
Why it happens: This fault occurs when the water inside the boiler gets too hot, triggering a safety switch to prevent damage. The most common reasons are a failed pump that isn't moving water effectively or a buildup of debris that has blocked the internal heat exchanger. It can also happen if there is a large amount of air trapped in the system or if the radiator valves are all closed at once.
Check that your radiator valves are open and not turned to zero.Check that your boiler pressure is between 1.0 and 1.5 bar on the gauge.Turn the control knob to 'Reset' until 'CL' appears on the screen, then turn it back to restart the system.
Your boiler's main control computer and the front display screen have stopped talking to each other, meaning the unit cannot function.
Why it happens: This fault occurs when the internal communication link between the boiler's main brain and the front control panel is broken. It is usually caused by a loose internal connection cable, a short circuit on one of the circuit boards, or moisture ingress affecting the sensitive electronics.
Turn the boiler power off at the fused spur switch (the wall switch).Wait for 60 seconds.Turn the power back on and see if the display restarts normally.
Your boiler has detected an issue with the sensors that ensure air is moving correctly, so it has safely shut itself down to prevent unsafe operation.
Why it happens: This fault occurs when the boiler's internal safety sensors fail to detect the necessary airflow required to safely clear exhaust gases. The most common cause is a blockage in the flue or a failure in the fan assembly, which prevents the system from confirming it is safe to ignite the burner.
Full guide for F5on the Viessmann Vitodens 100-W
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