Modern vehicles are very complex systems including several micro controllers and lots of electronics. This to make our travels safer, more comfortable, to help us navigate or just for plain entertainment. These systems demand a new generation of tools to verify correctness and to locate malfunctioning parts during development and maintenance. This Master's thesis is based on a device, the Mecel Flight Recorder, used in the vehicle industry for recording of messages sent on a vehicle's internal data buses. A remotely connected PC can be used to control this device. The main goal was to extend functionality to also include active diagnostics, which means that diagnostic questions are sent to the vehicle and the answer is captured and interpreted. Investigations of how to enable the diagnostic functionality and how to improve reliability of the remote connection have been done through literature studies and discussions with employees at Mecel AB. Most of the work has though been concentrated to implementation of demonstration software that enables the diagnostic functionality. The investigations have resulted in recommendations of how the remote connection can be improved using a combination of Serial Line IP, Cyclic Redundancy Checks and a new framing method. Also comparisons between different methods of enabling diagnostic functionality has shown that different solutions are appropriate in different situations. A few diagnostic applications that can exist on the side of the standard program of the Mecel Flight Recorder, have been implemented. The functionality of these applications has been divided differently between the PC based program and the embedded program. Since the programming was only to concern the embedded system most of the functionality was placed there.