Yes code reader is the place to start, in addition to code reader you need live data which will give lot of info on how your car is running. I have been using free HFM scanner
(http://www.peachparts.com//shopforum/showthread.php?t=236925) for my M111 and it has been handy software to have.

HFM ECU's are not very sensitive, you usually won't see a error code's for rough idle, minor air leaks etc.