Flatrater Diagnostics is pleased to bring you this excellent article written by Alan Gage.  Please note that it has been placed in a free access directory and is viewable by anyone. Please tell a friend who is not a subscriber! Note: PLEASE do not advertise for this article on iATN as that would violate iATN rules. Thank you Alan!

With all the Auroras that Jim has been working on lately I didn’t want him to feel left out, so when this one came in I figured it was my turn to share in some of the fun. This one is a ’95 with that DOHC 4.0 V8. It was originally in for some other work, which wound up being a bad shift solenoid. That resulted in the valve body being removed to replace it. I did the diagnoses on it and another tech wound up doing the dirty work. On his test drive after repairs the tranny worked fine but he said the MIL came on. When he checked for codes he found a lot of tranny codes stored so he cleared them and went for another drive. The MIL didn’t come back on, so it got handed back off to me to make sure there were no other problems.

I hooked up the Tech 2 and went for a cruise. After about 3 miles the MIL flipped on and I found code P0013 set for “Rear O2 sensor not ready”.  After seeing the code that set I took a look at the O2 signals on the way back to the shop and found the rear O2 sensor inactive and stuck at about 457mv. Take a look for yourself-

The front O2 is the solid line, and the rear O2 is the straight, dashed line.

I can remember that this code had also been set when doing my original diagnoses on the transmission. We called up the customer for approval on diagnosis and he confirmed that this had been an on going problem. Another shop (dealer of course ;-) had already replaced an O2 sensor to try and correct the problem. 

When I saw that the rear O2 was just reading bias voltage I was expecting to find a problem with the reference low circuit. It kind of seems like we’ve been here before-   I figured I’d just go ahead a raise the vehicle in the air and do my initial checks right at the O2 sensor. After I raised the vehicle in the air I began to search for the “rear O2 sensor”, and I couldn’t find the dang thing!

I couldn’t find what I consider to be a rear O2 sensor (behind the converter) anywhere. But when I looked up at the rear of the engine I was able to see the top of a shiny new O2 sensor. Apparently on this Aurora “rear O2 sensor” is a cute name for “bank 2 O2 sensor.”  Well, now that that is straightened out I guess I can continue on with the diagnoses.  I lowered the vehicle back down so that I could gain access the O2 sensor connector behind the engine.  Before I could even connect a DMM to the O2 connector I noticed that now BOTH O2 sensors were switching nicely on the scanner. Great!

I went ahead and back probed the O2 reference low line on the PCM side of the O2 connector and attempted to get the vehicle to act up. I wiggled and jiggled the wiring and dropped the engine in drive and reverse a couple times hoping to get the problem to re-occur. I had no success and was forced to go on a test drive while monitoring the O2 reference low line. Less than a mile from the shop as I was accelerating to speed on the highway it acted up again. Here’s shot of the O2’s at the time of failure and of the reference low circuit while the problem was occurring.

You can see that the rear O2 sensor was working normally and then all of a sudden just read bias voltage on the scanner. It also looks like the reference low signal at the O2 connector was fine.  Let’s take a look at the actual O2 voltage measured at the O2 connector while the scanner was reading a steady 457mv.-

And just so you know, NO, I didn’t take out the scope just to see if the O2 sensor was active. I’d normally just make this check with the DMM; but then you’d have to take my word on the activity, this way you can see for yourself.

So, the scanner is reading 457mv. The actual O2 signal is correct and varying when checked at the rear O2 connector. The reference low line appears to be o.k. when checked at the O2 connector.  What do you think and where are you going next?