Fault Tolerant Control Systems (FTCSs) are control systems including fault tolerant control. These systems are famous for enabling reliability, maintainability and survival ability in safe vehicle design. In some SCANIA Electronic Control Units (ECUs), the ECUs FTCS is based on a centralized fault detector to detect faults and a centralized reconfigurator to reconfigure the system with degraded performance rather than, for example completely shutting down the engine. However, with the size increasing in mechatronic system, the centralized architecture poses some problems in terms of performance, complexity and engineering facility. This thesis will present a Decentralized Service Based Architecture for FTCS. It is a hierarchy architecture composed of a completely decentralized fault diagnoser and a completely decentralized reconfigurator. The decentralized implementation in this thesis is exemplified on part of the Exhaust Emission Control 3 (EEC3) system, one of the ECUs of SCANIA. There are two main parts, denoted a decentralized diagnostic manager (DIMA) and the service based communication framework for the interaction between DIMA and reconfiguration. Compared to the centralized architecture, a decentralized action handler has been built locally in each software module so that actions can be activated as soon as the fault is detected, through which a fast and guaranteed response can be obtained. The concept Service means that the dependency between modules which is solely based on fault propagation. Service communication framework reduces the complexity of the original FTCS. Each ECU can be regarded as a node in the entire communication network of the mechantronic system in SCANIA, and once all the nodes are implemented with the decentralized service based architecture, Bayesian Network can be constructed to model the FTCS with uncertainties.