I personally use an Autophix ES910 scan tool. Lets me see live data streams, program batteries, etc. It was about $100. There are several options. I hear a lot of people using the more popular Foxwell tool or Carly. I think most people will tell you to get a BMW specific scanner before you throw parts at it. I have saved hundreds if not thousands of $ by using a BMW specific scan tool. If you use them correctly you can diagnose misfires before they throw a CEL. Not sure if it is on xoutpost, but bimmerfest has an excellent thread on how to use fuel trims to diagnose engine problems. I had a small misfire in mine that did not throw a CEL and this is how I was able to isolate the problem.