Bad fan connection

Check the wire connection between the fan and the Airdryer <A> driver board.

Blocked fan

Ensure that noting blocks the fan (wires, tubing, ...) of the Airdryer <A>

Malfunction fan

Replace the fan of the Airdryer <A>. See service procedure "Replacement of a large Air Dryer fan", or "Replacement of the Small Air Dryer fan".