190E 1992 Check engine light
 

Trying to help out someone out. This car has an O2 sensor light that is not lit but the CEL is on. Most probable cause? Can I take it to autozone or similar to check code, or is this a MB only diagnosis. I really do not know where to start, it is running fine, low idle perhaps the only thing I notice . HAs 128K on the clock

Autozone can only check OBDII diagnostics (1996 and later). There is some info here on the forum about making your own code reader with about $15 in Radio Shack components - for the W124. I assume it would work on yours too, but I don't know for sure.

My '93 doesn't have a check engine light, but the CFI Control Module still stores DTC codes and I can retrieve them using one of the homemade LED testers that has been discussed on this site.
Since yours has the check engine light then it probably has the built-in LED mounted in front of the battery. Press button for 2 to 4 seconds, note readout.

1 flash = No malfunction in system
2= Throttle position switch, wide open throttle contact
3= Engine coolant temperature sensor
4= Air flow sensor potentiometer
5= Heated oxygen sensor
6= not assigned
7= TD signal
8= Barometric pressure sensor
9= Electrohydraulic actuator
10= Throttle position switch, closed throttle position contact
12= Exhaust gas recirculation temperature sensor (California models only)

thanks SLK , will do