1999 ML320. When the codes call out a "left" engine problem, are they referencing the US driver's side? I had a 150/155 left O2 and replaced the driver's side which could have been a harder job but was involved! Anyway, I checked codes now and have a 1570 indicating a right disturbance in the force. I did start it with the driver's O2 disconnected. I get a bad feeling that the codes may be from the mechanic's perspective. For the record, the wiring plug pulled off easily; a 7/8" wrench unscrewed the sensor easily although the rotation was only about 20 degrees; pulling the wheel well liner gave me accesss to hook the wiring plug back up;, and tonight I'll crawl back under to push the plug back into the plastic neatness clip on the tranny. It would be OK to have to replace the passenger upstream O2 sensor too as I've got 210k but I will feel real stupid if I forgot the first law of Mercedes wrenching, if those **** ing Germans can take the simple and make it confusing, they will!