One objective of Industry 4.0 is to further enhance automation and data exchange in manufacturing among the whole value chain in order to provide new services and to decrease the use of resources. Industry 4.0 relies on communicating systems aiming at decentralized decisions for manufacturing requested products and services comprising humans and machines. The availability of systems and their data, e. g., manufacturing or monitoring data, online is an enabler of new applications and services including diagnosis. In this article we focus on the diagnosis aspect and provide insights into a methodology that allows for bringing in diagnosis of interconnected systems as a service. The methodology is based on system models and monitoring observations and thus is highly adaptable to changes in the system. In addition, to the methodology we give examples from applications of diagnosis for wind turbines and other engineered systems.