Your boiler's fan is running for a short period after the burner has turned off to clear any leftover gases safely.

Fan over run

What causes this fault?

This fault occurs when the boiler’s software detects that the fan is still spinning at high speed after the demand for hot water has ended. The most common reason is a communication error between the fan and the control board, where the fan fails to slow down to a safe standby speed. It can also be caused by a sticking signal wire or a fault within the fan’s internal electronic motor.