I'm no pro - but I do have a 93 = 190= 2.6 that is having some smog issues. A wiser member than I, suggested that the first thing I should, was to sure the secondary ignition circuit ( coil ,thru the Cap and down to the plugs) was perfect.

That made a huge difference, but still did not allow me to pass.... I'll do the O2 sensor next. With the M103 motor in a W201, be sure you heat it all up well before the test, that is, don't go in cold...try to have at least 20-30 mins of running prior to the test Your Cat from is pretty far from your exhaust manifold, the heat will help with a cleaner burn, and hopefully better emissions.

__________________
1993 190E 2.6 135k
1989 Ford Thunderbird SC 5 Spd 79K