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 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 F5Your boiler has detected a fault with the sensor that monitors the water temperature returning to the unit, causing it to stop running for safety and efficiency.
Why it happens: This fault occurs when the sensor responsible for measuring the temperature of the water coming back from your radiators fails. The most common reason is an internal electrical failure within the sensor itself or moisture ingress that has damaged the connection, preventing the boiler from accurately monitoring the system's heat levels. Without this data, the boiler shuts down to prevent inaccurate firing and potential damage.
Full guide for 60Your boiler has lost its connection to the sensor that measures the water temperature coming back into the unit, which often results in the system shutting down for safety.
Why it happens: This fault occurs when the boiler’s internal computer loses the electrical signal from the sensor that monitors the temperature of the water returning from your radiators. The most common reason is that the sensor itself has developed an internal electrical failure or the thin wires connecting it to the main control board have become loose or damaged. Because the boiler can no longer verify if the water is at a safe temperature, it shuts down as a precautionary measure.
Full guide for 68Your boiler has stopped working because the water pressure inside the system is too low to operate safely.
Locate the filling loop (usually a silver braided hose under the boiler)Open the valves to allow mains water into the system until the pressure gauge reads between 1.0 and 1.5 barClose both valves tightly and restart the boiler
Your boiler has stopped working because the water pressure inside the heating system has dropped too low.
Locate the filling loop (a silver flexi-hose with two valves) under your boiler.Open the valves slowly until the pressure gauge on the boiler shows between 1.0 and 1.5 bar.Close both valves firmly and restart the boiler.
on 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
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
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