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MB-Dude · #6 11-10-2007, 12:45 PM

Well… replaced the front O2 Sensors with new Bosch (p/n 13638) units from AutoZone. Unfortunately, there is no change in the ECU status. At over $300 for sensors and tax, this was a very expensive experiment. Guess that’s what I get for wanting to install OE parts as opposed to cheaper 'universal' components. Anyway, for reference I have attached a pic of the old O2 Sensor (top) sitting beside the new sensor (bottom). As you can see, the old sensor looks pretty clean, with very little soot or residue on the probe head.

OK, back to the problem… In my original post, I attached a screen short of the Engine Monitor. That screen is still identical, even after the O2 Sensor replacement and a 20 minute drive. While this is not long enough to complete an infamous ‘Drive Cycle’, it should be long enough to clear the errors noted - both Fuel Systems are still reporting “CL-Fault: Closed Loop, but fault with at least one oxygen sensor. May be using single oxygen sensor for fuel control.”

So I set up the OBDII reader to capture real-time O2 Sensor output. The attached pic is a screen shot of that process. Each graph represents 30 seconds of time duration. You will note that Bank1, Sensor 1 (upper left graph) seems to be oscillating somewhat normally, while Bank 2, Sensor 1 (lower left) is flat, at zero volts. The other 2 sensors are post-cat and are steady as a rock at ~0.5 volt - at least the cats are doing their job.
If I accelerate the engine up to ~2000 RPM, I do see a slow, steady change in both post-cat sensors, and by the same amount. Thus, they seem to be reporting and responding correctly.

But why is Bank 2, Sensor 1 reading zero volts? Going back through previous notes, I see the exact same output BEFORE the O2 Sensor change out. Thus, the O2 Sensors are not the root source of my problem.

If I watch the O2 Sensor readings long enough, Bank 2, Sensor 1 will jump inexplicitly to the normal range, then drop back to zero volts. I am interpreting this as an intermittent signal. Based upon this information, I need to trace signals.

QUESTION: Can anyone help me identify the signal path from the pre-cat O2 Sensors to the computer? Cable connections, wiring routes, etc.

However, I believe the problem is more electronic than wiring. Remember – my CEL is *not* lit and the engine runs incredibly strong. Also, the sensor that is post-cat for Bank 2 is operating normally. This suggests Bank 2, Sensor 1 is actually operating correctly, that the Bank 2 ECU is receiving the data and adjusting fuel accordingly. For some reason however, the computer(s) are not ‘reporting’ the correct O2 Sensor signal. This, I believe, is what is causing the Fuel Systems to report “CL-Fault: Closed Loop, but fault with at least one oxygen sensor. May be using single oxygen sensor for fuel control.” This in turn, does not allow the emission stuff to satisfy; causing the ECU’s to remain ‘Not Ready’.

At this point, I am open to any and all ideas, suggestions and recommendations. Unfortunately, my R129 Repair DVD is in transit to New Jersey for upgrade from MB, so I’m really blind here.

Cheers,
Jeff