The purpose of this master thesis is to contribute a service-based decentralized architecture for Electronic Control Unit (ECU) with fault tolerant control. As ECU systems are becoming large-scaled, centralized-architecture fault tolerant control is facing challenges in performance, complexity and engineering, for its dependencies, non-modular, non-scalable and so on. In Scania’s ECUs, the architecture is applied by a centralized diagnose system. In this thesis, an alternative solution – service-based decentralized architecture is presented. In the architecture, both diagnostic and reconfiguration are completely decentralized in modules of ECU system. Modules can be seen as a software component with self fault tolerant control abilities, which provides different quality of service to system. A purely service view of ECU system can be built as the foundation both for fault tolerant control architecture and for modeling control system. The thesis project is, firstly to implement a general decentralized diagnostic platform which can be called by modules of Scania Exhaust Emission Control system (EEC3) system. A centralized communication node is constructed for off-board diagnosis by PC-tools. Lastly, service based architecture is built in EEC3 PUMC module as service provider.