Mini R50 1.6 Cooler fan does not run

Servus dear motor-talkler, Unfortunately I didn’t find anything in the Sufu, so I need help from you. With my mini year 2003 1.6 90Ps the fan does not start from the cooler any more since the cooler has been changed.The display is always in the middle even when the water boils.. Backup, relay checks. When I pull the plug from the coolant sensor the fan starts immediately. Delphi gives the error code P1661 and 1481 but I did not find anything really helpful. Thanks already in advance!