Just bought a 93 325is from a friend. Took it to get smogged and it didn't pass. Just got a new 02 censor and new catalitic converters put in as well and the "Check Engine" light keeps coming on and the code on smog keeps saying it's the O2 censor. My mechanic works on nothing but BMW, Mercedes and Porche's. He's about to lose the last of his hair trying to figure out what's going wrong. Anybody been there or done that?