I just thought I’d share my experience of getting the On Board Diagnostics working for my early Mercedes R170 auto 1997 in Imperial Red (UK model with 38-pin diagnostic connector), all for next to nothing!

On older R170s the socket is under the bonnet, as pictured:

From obd2warehouse.fr I purchased a “MERCEDES 38 PIN OBD to ODB2 16 PIN CAR FAULT CODE DIAGNOSTIC ADAPTER CABLE” or “38 Pin to 16 Pin OBD2 Female Adapter Connector Diagnostic Cable for Mercedes” – or something like that, descriptions vary, but there’s one pictured below.

Then I bought an Autel MaxiScan MS300 CAN OBDII scan tool, though I actually bought this for a modern car, it appears to work in conjunction with the 38 pin adapter below. The MS300 code reader is very basic and out of date currently I thought, you can choose  other AUTEL alternatives.
Well, all I can suggest is that it was a bit of a fiddle to get the 38 pin plug to seat properly in the car’s socket. Do not push it so hard, though care is needed, it is all a little fragile.
Then I bought a “ELM327 OBDII V1.5 CAN-BUS USB Auto Code Reader Diagnostic Interface Scanner” – again several titles are used, which comes with a little CD (which can be a problem, I only had one PC that would accept it). Loading the software on an old Windows 7 netbook was pain free, and you end up with a vaguely amateurish open-source program which displays the data. (I would NOT recommend updating this to a newer version from internet, this did not work for me) – oh and the program is sadly incomplete, it has menu items that don’t work. Though… you do get the source code too, so if you’re software savvy you might be able to DIY!

so, the MaxiScan MS300 gives you a small amount of info, really just the codes and maybe the ability to switch off a check light.

the ELM327 scanner in conjunction with a laptop gives you much more info, and can be used with the engine running to get ‘real time’ data.