Your boiler is currently running its internal pump to circulate any remaining heat after the burners have turned off, which is a normal part of the cooling process.

HTG pump run on

What causes this fault?

This fault occurs when the boiler successfully lights but cannot confirm that the flame is still burning while it is running. The most common reason is a worn-out or carbon-coated sensing probe that can no longer detect the heat of the fire, though it can also be caused by an incorrect mixture of gas and air.