what is the voltage at the green wire when key is on and sensor unplugged?

any change in sensor reading after unplugging the 2 pin heater?

the unplugged sensor should make the duty cycle freeze at 50%. good luck, chuck.

First to answer some questions. Disconnect of the heater wires had no effect on the measured signal from the sensor. Thus no cross-noise present. Next I verified the chassis-engine grounding and it proved to be very good.

Next step was to remove sensor and propane torch test it. The propane bench test repeated the findings of negative readings: -0.1 to -0.9. The sensor "battery" has shifted its "ground" reference by approx -1.0 volts.

A negative sensor signal would drive the CIS-E to a hard rich condition and this is the problem I experienced. This was causing engine RPM reduction and power loss to the point of stall.

I have never seen this failure mode reported for an O2 Sensor but I am not an expert here. Any Bosch technologist out there care to confirm that this does occur and indicate how/why it occurs??. Also could the computer module have been designed to be smart enough to recognize this condition (continuous hard neg O2 sig input in presence of high RPM and at operating temp with forward motion) and force it to the limp home mode. What it did could have caused a major catastrophe were I in the Lincoln Tunnel or other such compromised situation. If Bosch wants the failed sensor I will send it to them. I have verified "limp home" mode by pulling the input lambda sig to the EHA. That is what I am running with now. What I got from the neg failure mode was NOT limp home....it was a complete stall. This is a drive critical and a safety concern. IMHO an O2 sensor should NEVER send a vehicle into a stall. Let me know if I am alarmist here.

More to come after I install replacement.......

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Dionysius

I can't say what would happen if the O2 would go negative as I have never seen it. I have been working on these types of problems since around 1980 when most domestic car started using them and I have never seen a negative voltage come out of an O2 sensor, but one thing I did learn is "Never say that is impossible, because if you do, you will be proved wrong". Again most of my experience is with domestic and very little on euro cars, but most cars used Bosch sensors, like yours. A domestic car won't go into closed loop mode until the O2 sensor shows activity of some sort. I would think staying at or below 0v would keep closed loop from happening.

If you reconnect the EHA but unhook the O2 will the car run reasonably well?

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84 500 SEL (307,xxx miles)