I've been trying to clear my OBDII codes on my '96 C280 for a while with no luck. First I tried my neighbor's Mercedes C-class specific scan tool (Unitec??), but for some reason it didn't work on a C280 (the instructions list every model of C-Class except C280!?!). Then I ran down to Autozone and rented their scanner tool, which could read the codes, but not erase them.

This has happened several times in the past when independent garages aren't able to reset the codes after completing a repair. And, since my state now requires the OBDII check as part of the annual emissions certification I end up having to take the car to the local MB dealer to reset the codes, where they proptly charge me $100 for 2 minutes of work.

Any ideas as to why I can read the codes with a standard scan tool, but cannot erase them? Any older model year W202 owners had any luck with cheap scan tools (just need to read code and erase)?

Ideas are much appreciated.
Thanks.