The objective of the Automated Manufacturing System reconfiguration is to react to failures such as breakdowns of actuators or sensors. Following such events, the control laws run by the control system are often blocked and thus become inadequate. Thus, it is therefore necessary to reconfigure the control system. This reconfiguration process is inevitably based on the capacities still offered by the operating part to achieve the production objectives. In this context, the paper proposes an on-line function diagnosis able to provide the required information on the capacities of the operating part. It is based on a model of the operating part in normal operation, and generic rules to obtain the possible origins and consequences of a detected symptom of failure.