I bought the OBD2 unit from these guys. You buy the adapter and download the software from the internet.

http://www.obddiagnostics.com/

You get all the DTC codes and a real-time read-out of sensor voltage and correction along with short- and long-term fuel trim, manifold pressure, etc. You can also clear codes, and get a "ready to test" reading. Not WIS but not bad for 75 bucks.

Obviously you need a notebook PC if you are going to use it while driving.

It has a kind of cool feature where you can record all the readings taken and play the session back.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe