As Larry stated, I think you could find value in the article I wrote. One important thing I think it states: Check the duty cycle,key on engine off, first. You should get 70% if its a federal car and 85% if its a CA car. The point to this is that it confirms your hook up.
If you have a fixed duty-cycle then you are reading a realtime fault code. Don't have my book to tell you what 70% would be.
Bosch Master, ASE Master, L1
33 years MB technician