How do you retrieve error codes on pre OBDII cars?
 

Alright, I got this 1991 190E 2.6 on the straight and narrow again with the exception of a check engine light. My guess is that with 108K miles and a 60K service interval it is likely an O2 sensor. How the feck do I figure this out with out the use of the old school diagnostic tool or am I out of luck?

The model year is everything.

Sorry, 1991. Is it really that much diff. for all pre 96 MB's?

Yes, big changes over the life of the 190E, from virtually nothing to at least something towards the end. Not sure if you have an 8 pin or 16 pin diagnostic connector but you just need to build the code reader and pull the codes from pin 3. Post 3 here - http://www.peachparts.com/shopforum/tech-help/107630-e320-16-pin-dtc-charts-inc-pin-8-pin-14-a.html for building the code reader. You need the CIS 103 motor code list.