You need to probe the O2 sensor AT the sensor!!! In the 300E, you pull up the front passenger carpet and the wire was right there on the floor pan. Disconnect the sensor at the quick disconnect fitting, probe the sensor wire (either the only wire on older sensors or the "non-white" wire (the white wires (two of them) are the heater wires) and ground the other wire. Start the car. The sensor should always be providing a voltage! It will read rich (a lower than normal voltage I believe) until the car kicks out of "start-up" mode. That's when you start adjusting the Fuel/Air ratio. P.S. Keep count of the direction and amount of turns so you can get it back to where you started!!!!!!!!! AND Write down each adjustment.