BoilerFaultCodesThe 6 most common faults
Your boiler is struggling to light its flame, meaning your heating and hot water will likely stop working soon.
Why it happens: This fault occurs when the boiler's control system attempts to start the heating cycle but fails to successfully establish or detect a flame. The most common reasons include a worn-out ignition spark, a faulty gas valve not releasing fuel, or the internal sensors becoming too dirty to 'see' the flame once it has lit.
Check that your gas pre-payment meter (if you have one) has creditEnsure your gas stopcock is fully openPress the Reset button (backwards arrow or 'Flame' icon) for five seconds
Your boiler is trying to start up but the internal fan is not spinning properly, preventing the heating from firing up.
Why it happens: This fault occurs when the boiler's control system sends a signal to the fan to begin spinning, but it fails to reach the required speed to safely start the ignition process. The most common reasons are a build-up of debris or moisture inside the fan casing, or a communication failure between the main control board and the fan motor.
Full guide for S.1Your boiler has stopped working because the water pressure inside the heating system has dropped too low to operate safely.
Locate the external filling loop (a flexible metal hose) connected to your heating pipes.Slowly open the valves on the filling loop to allow water to enter the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (usually between 1.0 and 1.5 bar).Close the valves tightly and check the boiler display to see if the error clears.
Your boiler is temporarily pausing heating because it thinks the house is warm enough or to prevent the system from getting too hot too quickly.
Why it happens: This code appears when the boiler reaches its target temperature and briefly pauses to prevent the system from overheating or wasting energy. It is often caused by the heat not leaving the boiler quickly enough, which can happen if your radiators are turned off or if there is a minor restriction in the pipes. In many cases, it is simply a sign that the boiler is waiting for the system water to cool down slightly before firing up again.
Check that your room thermostat is set to the desired temperatureOpen all radiator valves fully to ensure heat can move away from the boilerWait up to 20 minutes for the 'anti-cycling' timer to finish and the boiler to restart automatically
Your boiler has stopped working because it has lost the connection to the sensor that monitors the temperature of the water flowing through your heating system.
Full guide for F.0Your boiler has lost its connection to a temperature sensor, meaning it cannot correctly measure the heat of the water returning to the system and has safely shut down to prevent potential damage.
Full guide for F.1on the Vaillant ecoTEC 65kW
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 Vaillant 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