Re: Engine Warning Light
The one I've got is a USB pod that plugs into diagnostic socket, which is adjacent to the fuse board under the dash just above accelerator pedal.
It's supplied with a 3m USB to connect pod to laptop and comes with software to diagnose and reset warnings. It also allows your monitor in real-time and/or log engine parameters such as RPM, temp, O2 etc etc. Pretty much anything that the ECU sees, you get to see through this software. Some software even allows you to do things like run acceleration/performance tests.
Makes you realise how much you are being ripped off when you go to garage and they charge you £30-50 just to plug it in, never mind fix what it comes up with.
Keep in mind that these cheap units will only interrogate the Engine management functions and not anything else like ABS or Airbag. Also will probably only work with 2001 onward models. 2003 onwards may need a more advanced model of diagnostic tool that supports CAN protocol as well as ISO/KWP2000?
Hope that helps.