Your 98 SLK is an R170 right?

If so that has a lot of the same electrical systems as the W210.

The problem with these older models is that the individual computers do not go back to the OBDII port. Each module has a dedicated pin on the 38 pin plug under the hood for its fault diagnosis.

Only the federally mandated minimum went to the OBD port on these models - mostly emissions related data.

When my 210 got hit last winter in the door, the body shop disconnected the door air bag while the battery was still connected. This sets a permanent fault in the air bag module.
They had a fairly sophisticated scanner but could not clear the air bag fault so I had to take it to the local MB dealer.

In less time than it took me to finish my cup of coffee in the waiting room they cleared not only the air bag fault but a bunch of other cob webs in the system with their Xentry computer. I got a 4 page print out of old faults that had been registered.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech