Unified Diagnostic Service and On Board Diagnostics require a client side device with necessary software to implement certain specific algorithms. This paper proposes a highly optimized and generic model based architecture to implement client side algorithms used in Unified Diagnostic Service systems and with On Board Diagnostics which can be reused for any hardware target. The proposed method can implement particular algorithms which include flow control, timing control, database parsing, logging of messages, diagnostic database parsing, security unlock, intuitive HMI layer, DTC display with textual information, frame control, multi network - multi ECU support, software flashing, physical-functional message handling, and interfacing for multiple hardware host devices. Re-usability of this model based product ensures that it can be ported to the diagnostic tool used by a work shop engineer or by a diagnostics validation engineer working at OEM or Tier 1suppliers. This means that this software is hardware independent. Scalability is ensured by designing the model in a modular way, thereby the accommodation of additional logic is made easy. The model also supports automated diagnostics implementation using automation scripts which can change model variable values in real time. The aim of this paper is to propose an efficient software architecture for the client side programming which is memory optimized, reusable and scalable. In this paper reusability refers to the ability of porting the same diagnostic client application software to different hardware targets.