V12 ECU's Reporting 'Not Ready' - Page 3 - PeachParts Mercedes-Benz Forum

MB-Dude · #1 11-02-2007, 02:33 PM

Although the vehicle in question is a ’96 SL600, this situation may be applicable to other V12 models. About 6 weeks ago, the CEL came on when the engine began running quite rough, very poor acceleration, etc. The problem – whatever it was – cleared itself for the very next time I started the car (to begin troubleshooting) the engine ran incredibly smooth. A test drive revealed the engine was again running as good (quite strong) as it had before the CEL light. What a mystery.

I use an OBDII Scan Tool made by ProScan (www.myscantool.com) and it showed the following Trouble Codes (listed in order as retrieved) from the SL’s memory:
P0307 – Powertrain: Cyl 7 Misfire Detection
P0309 – Powertrain: Cyl 9 Misfire Detection
P0308 – Powertrain: Cyl 8 Misfire Detection
P0300 – Powertrain: Engine Misfire Detection
No other codes were listed and the CEL was, of course, registered as ‘On’.

Because the engine was running smoothly and I had documented the codes, I reset all codes and the CEL. And because of CEL activation, I expected the ECU’s to be in a ‘Not Ready’ state, and in fact, they were ‘Not Ready’. So I figured the ECU’s would settle down after an appropriate length of a Drive Cycle routine.

Here’s the problem… Even after 6 weeks and over 120 miles of mixed residential, city and highway driving, the ECU’s still register ‘Not Ready’. The attached file is a screen capture showing the reported state of the components. In addition, the misfire problem has *not* come back (this is good) and excellent vehicle performance has been there.

My question/concern revolves around two comments on the left-hand side of the screen shot: “CL-Fault: Closed Loop, but fault with at least one oxygen sensor. May be using single oxygen sensor for fuel control.” There are two entries, one for each bank of cylinders. Does this entry make sense? Are the ECU’s sophisticated enough (1996) to ‘fall-back’ to single O2 sensor operation, if a fault is detected in the other one? For information, there are a total of 4, Oxygen Sensors in the exhaust. Two per side/bank of cylinders; one each of these before the cats and one after the cats.

To my knowledge, the O2 Sensors have never been changed and the vehicle has 105,000 miles on it. I have always heard mixed thought on whether Oxygen Sensors should be changed periodically, though I have never had to change one. I know they are easy to change; just never had an occasion to change one.

Any and all thoughts/comments on the above problem would be greatly appreciated.

Cheers,
Jeff