Today, I replaced the MAP Sensors for both Banks. No change in status - Bank 2, O2 Sensor 1 still reports 0v output and, of course, the ECU's are still 'Not Ready'. Both fuel systems report closed loop, but a fault with at least one O2 Sensor.

Question #1: The Oxygen Sensor used in this vehicle (Bosch p/n 13638) has four wires. Presumably, 2 wires are four the heater and 2 wires are for the probe. Can anyone confirm which wires are which, or point me in the right direction to find the information?

I want to begin moving along the wiring harness connection-by-connection (the word I’m think of rhymes with ‘yuck’), until I get to the ECU connector. First place to start is at the O2 Sensor itself.

Question #2: If the heater for the O2 Sensor were not to energize for whatever reason is there enough heat in the exhaust to activate the sensor?

I suspect so. However, I have been reading the Bosch Fuel Injection & Engine Management book and one of the things mentioned is that O2 Sensor output is zero if not enough heat is present. The only contradictory observation is that I have monitored both O2 Sensors when the engine is cold. OBDII reports Bank 1 at 0.5v and Bank 2 at 0.0v.

Out of desperation, I even swapped the battery with my other SL, and even the bullet-proof W123. Obviously, that was an exercise in futility.

Does ANYONE have ANY ideas? Anything? I feel I’m wasting my time documenting all this.

Cheers,
Jeff

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Couple of things. If you have swapped the pre-cat O2 sensors, and the bank 2 sensor works OK on bank 1, then you already know the O2 sensor is fine. There should be plenty of exhaust heat to heat the O2 sensors to working temperature even if the heater circuit isn't doing it for some reason. But, if there was a short or open circuit in the heater, the ECU should detect it and give the fault code for it and a check engine light.

Good idea to monitor the sensor signals from cold. I should do that.

Thanks for updating this thread. I will, too, but I don't have much time to spend on it right now. I've got my high idle problem to deal with, too. I'm guessing it'll be best to solve that one first, then get back to the O2 sensor.

I'm not so sure that your flat O2 sensor signal has something to do with the lack of "readiness" indication. My diagnostics all show "complete" despite having the same bank 2 pre-cat behavior as you.

A couple of things to try: 1. Unplug the pre-cat O2 sensors one at a time while the car is running (carefully, of course) and see what happens to the signals. I did this, and the good bank 1 signal flat lined at 0.46 V. And, I'm not 100% sure at the moment, but I think the bank 2 signal did the same thing (went from flat at 0.0 V to flat at 0.46 V), which means to me that the ECU is getting a signal from the O2 sensor, and the 0.0 V signal may be real. I need to do this again to confirm and take better notes. 2. Check the temps of the bank 1 and 2 exhaust pipes with an IR thermometer and compare. Again, I can't recall exact temps, but I think bank 1 was running around 320 deg. C post cat, and bank 2 was around 60 deg cooler at around 260 deg C post cat. That struck me as a big difference, but again I need to confirm and take readings pre-cat, when I get a chance. And, I really don't know what that means at the moment, but there it is. I pinged the coolant pipes from each cylinder head at the front of the engine, and both were pretty much identical in temperature. There doesn't seem to be an issue with different head temps.

You know, the thought just struck me that maybe it is time to revisit the stuck open air injection valve theory. If there is significant air admission into the exhaust stream, there may also be the significant cooling of the exhaust gasses as I just mentioned. But, I have no idea if air admission via the injection valve could be enough air.

Brett

addendum... So I checked the O2 sensor signals before I started the car this morning. Ignition on, engine not running, hooked up OBDII reader. All sensors except bank 2 sensor 1 read flat at 0.46 volts. Bank 2 sensor 1 read 0.0 V. It appears that the ECU reports a flat reading at 0.46 V when the sensors are inactive. So I think I was incorrect when I said above that disconnecting bank 2 sensor 1 while the engine was running caused the bank 2 sensor 1 signal to go to 0.46 V. I think it stayed flat at 0.0 V, but I need to do this experiment again.

If this were a wiring problem, it seems to me that a broken wire or bad connection would be just like an inactive sensor, i.e. the ECU would report a constant value of 0.46 V as I see with the other sensors when they are disconnected or when they are not functioning because they are cold. I'm starting to swing toward the ECU outputting a bad signal to the diagnostic connector.

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1997 S600 sedan