Talking to W202 owners, the most common reasons for CE lights are:
1. Misfire codes from aging sparkplugs. I seem to be one the few people I know that changes plugs during the car's lifetime.
2. Low voltage codes from aging batteries. A recharge might do the trick, but after five years a new battery might be in order.
3. MAS. All OBDII cars seem to be having MAS problems, due to the nature of the device. Mercedes is not excempt from this. Luckily, their MAS is a reasonable price. (Mazda 626 is $1300 part alone)
Steve is right (as usual, again, of course) about the OBDII codes being able to be read by non-MB shops. Wasn't that the whole premise behind OBDII? My understanding was that it was related to "universality."
1998 C230 "Black Betty"