The functionality of a designed product is realized by the emergent behavior of its components. Regarding mechanical products, the corresponding component interfaces are often realized as contact surfaces. To improve a products functionality in a new product generation it is therefore important to identify these contact surfaces. This normally happens with manual models like the contact and channel model for design improvement, but there is a potential to automate this step as the information on parts contact surfaces is implicitly modeled within the CAD model of a products assembly. In this publication, an algorithmic approach is defined, implemented, and tested to achieve this objective. Eight requirements are taken from a workshop with ten engineers. Based on the programming interface of a widely used CAD software, contact surface pairs are then identified automatically for further design processes to support designers in improving product functionality. The algorithm is evaluated with two use cases consisting of a model of a drive shaft and a model of a manual screw jack.