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)...

The wire at the test point should be Pink, but in any case that .083V sounds very low (lean mix) if that's really the sensor output (as opposed to some related level in the AMM).

I think it's better to meter the sensor signal directly, where the Black lead connects to the fat Green lead to the ECU. Peel the rubber boot back to connect meter + lead and meter ground to the valve cover braid.

Ideally with warm engine and hot sensor the voltage should be hard to read, rapidly bouncing in the 0.2 to 0.8 range. Hopefully most readings will cluster around 0.3—0.7, with an occasional 0.2 or 0.8 and maybe a rare 0.1 or 0.9V.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Hi,

The reason that you aren't getting a proper reading is because you don't have the proper equipment. If you hook up an oscilloscope up to the test point referenced to ground....

I was using a Fluke DMM referenced to ground. Nothing AC. The meter is quick enough to measure the coil being grounded by ignition computer while running (~ 15 revolutions per second x4) which reads about 9VAC. I guess I could try my old analog unit ...

The sensor output is a black wire.