K, first of all, the test was the Exhaust Gas Analyser and it was the O-2 reading that was off the wall with the O-2 sensor connected and dropped significantly while disconnected.

Jay, I'm sorry to hear you are experiencing the same symptoms. Even my auto instructors are perplexed but they keep plugging at it. And these guys are the best.

As far as the relationship between the O-2 sensor and the CAT are concerned....on the four cylinder engines there is one O-2 sensor located on the head pipe while the cat is located just after my flex pipe which is relatively half way between the head pipe and exhaust pipe. I know that some engines....(mostly late model or larger engines I think) have 2 O-2 sensors...located just before and just after the cat.

You do hit on something when you mention the "default value" that the ECM will revert to. This value is referred to as the "limp-in" mode which basically sets the ECM to basic values so the truck will run without the O-2 sensor reading in order for you to 'limp-in' to a service station.

There is also something known as the "open-loop" and "closed-loop". On cold start the truck is running in "open-loop" meaning that basic values are set because certain sensors have not reached operating temp and cannot yet send their signal to the ECM, and the air-fuel mix is rich. Once the sensors have reached operating temp the sensors send their signal and the ECM adjusts the air-fuel ratio accordingly. I had thought this might be the problem because the truck runs great on cold start and then once it warms begins to run like crap but it does this with or without the O-2 sensor connected.

Anyways, thanks for writing in Jay and I will keep this thread posted in my progress. Today, it's back to the drawing board. My instructors won't let me give up and send it out...there's really no need I guess. One of them owns a service station but look at the money I am saving do this on my own again, and I get experience and lab credit too!