So I decided to test my O2 sensor using the purple wire in the blue plug which is located by the coil on my 86 240. I often wondered where this "Lambdasond testpoint" was located, which is shown as being an output of the LH ECU, until one of the green manuals told me where is was.

I measured .083 volts DC. No voltage swings - nothing AC. I pulled the vacuum line on the fuel pressure, no change in voltage. It was measured at idle while warmed up.

What seems strange is the car runs fine. I've measured my highway MPG at 25 several times. I've started to doubt the importance of this sensor in the role of fuel management other than providing the trim for the injector cycle time. Maybe the ECU is smart enough to ignore this bogus feedback?

So my next step is the verify I have 12V to the heating element (small white wire), that is has a ground connection (other small white wire?) and measure the output at the sensor itself (green wire)...