Hello. My 1997 C230 kept giving the code for a bad O2 sensor (and a constant Check Engine Light) despite repeated replacement of the sensor with OEM equipment. My mechanic couldn't figure out what was wrong, and recommended I take it to a dealer.

I resisted the dealer suggestion, as the cost might have exceeded the value of the 15 year old car. Luckily, I found a very solid Mercedes mechanic who runs his own little shop. After careful monitoring, he detected that one of the prongs in a connection had gone bad, causing the O2 sensor not to function. After soldering in a replacement prong, the problem was solved. A $300 solution.

Just thought this might be useful information to someone out there with a similar problem.