Maybe others will post with their experiences.
I'm using an interface box that plugs into my ODBII connector and my laptop... runs the tests and displays the info on the computer display. Bought it over the net complete with Windows software.
Works without flaws on my 1996 S500. Reads all the O2 sensors, fuel trim, etc, etc.
For the original poster....
The diagnostic connector under my hood is not ODBII, it is Mercedes specific. Is your underhood connector ODBII?
Fed law states ODBII connector must be near the steering wheel inside the car.
Hope this helps. KenP