I've just successfully replaced the O2 sensor in my CIS-E Motronic ladened Passat, which has made an unbelievable difference in my etest. First time failed, second time passed. Why?? well I learned a little something...
The generic Bosch 3 wire sensor sold by VW uses a 4 wire connector plug. When you buy a non-VW specific sensor and have to splice the wires together you might be missing out on the fact that the 4th wire coming from the ECU wire harness is key. The Bentley wire diagram shows all 4 wires from the ECU going to the 4 wire connector. The signal is wrapped and shielded along with the ground wire, while the O2 sensor heater wires are kept separate. When the VW specific O2 sensor is installed, the only wires leading from the connector plug to the sensor are the signal and two heater wires. The fourth wire (ground) has a wire that comes straight out of the O2 sensor 4 wire connector plug about 3 inches, and ends with an eyelet to be attached to a ground point on the rear motor mount (at least on the Passat). It has been explained to me by a friend who knows more about electronics than I, that this ground wire plays a very important role in shielding the signal wire from excessive noise, allowing for a clean path from sensor to ECU.
Now previously I had installed the generic 3 wire sensor and spliced the heater and signal wires together and left this ground wire capped and loose in the harness. When trying to adjust the DPR current for optimum performance I would often get really wacky responses, and never had the same reading twice from day to day. Since my initial emissions test was pathetically failed I gave in and though that my generic 3 wire sensor just didn't do the job as the "factory" sensor and I spent $170's on an actually Bosch sensor from VW. Would you believe the sensor I was provided at the dealer had the same Bosch part number on it as the generic one So no difference there, but I did hook up the wires including ground and now find incredible difference in drive-ability. So when hooking up your generic Bosch O2 3 wire sensor, remember to attach the ground wire in the ECU harness to the ground point at the rear motor mount.
Also: I followed Scirocco16v's CIS-E fuel Mixture write up and found it very helpful. However for those really into the DPR adjustment for uber fine tuning, I wish to share with you something that Sciricco16v fails to mention on his website. When establishing a baseline for your DPR current, I discovered that I could get much more than 1 mA difference in readings with having accessories turned on. In fact 3 mA leaner was seen on my car when comparing the DPR current with the blower motor turned to the #3 position, and the rear defrost turned on and having those same accessories turned off.
But I still passed my etest, so its all happy days! Rev on.