Computeractive magazine, sept-oct 2007, had a good article by Mark Whitehorn about plugging your car into your laptop in order to use the car’s computerised fault diagnosis system, like the garage does. You can find the full article, Get Back On The Road, in the online library Highbeam Research and access it on a free trial subscription – http://www.highbeam.com/doc/1G1-168898275.html/
According to it, a number of cars have had information-gathering Electronic Control Units since the mid-90s; all US cars have been required to conform to the On-Board Diagnostics II standard since Jan. 96; all Europe-made petrol cars have had to follow the EOBD standards since Jan. 2001; and most European diesels have conformed since the start of 2004. The European standards mean all cars must have an easily accessible standard 16-pin connection to their ECUs.
Websites including www.troublecodes.net and www.peacockandpurvey.co.uk list the fault codes and their meanings and there is advice to be found – which Computeractive did not test – on making rough-and-ready diagnoses by plugging into the ECU connector and watching the engine lights flash.