1996 Pontiac Grand Am P0140

Hi to both of you! My name is Chip and I am a disabled ex-class A Tech. I have started a ministry out of my church where I work on (and hopefully fix!) peoples cars for no labor charges. Through donations to the ministry we even purchase the needed parts if the owner is really destitute. Eventually I hope to take in donated cars that need repairs and, after making them roadworthy, re-donate them to people who otherwise can't get a job or be self sufficient.(We have no mass transit here) I'm finding it to be a really rewarding experience to see the looks on their faces when I'm through and there's no bill. Now I have a question for you, if you would be so kind. I'm working on a '96 Grand Am with a 2.4 and about 126k that when I first got it was blowing BLACK soot and smoke out the tailpipe. It had a code of P0140, no activity on the downstream O2. After replacing a bad fuel pressure regulator and the plugs, I replaced the sensor, since it was so covered in carbon it never would have worked. The car now ran fine, mileage doubled, and we were all happy. I did tell them that the converter may have been damaged by running it for a while with all that excess gasoline going through, and they said they had already replaced it. I somehow didn't ask if it was replaced before or during the black smoke. Now comes the fun part: I got a call back from them that the CEL was on again, although the car was still running fine. I pulled DTC P0140 again, removed the sensor thinking that some remaining carbon had fouled it again. It appeared to be in good shape but since it was under warranty I replaced it again, then watched it on my scan tool. The voltage was at .440VDC which is in the right range, but it never changed even a thousandth of a volt, under any condition of load or throttle. I unplugged it from the harness and the reading STILL said .440VDC! It didn't set a code or even flash the CEL or affect the running at all. I don't see how the scan tool could still be seeing a voltage reading when the source of voltage was removed. I checked the continuity of the signal wires to the PCM and for any shorts to power but could find nothing wrong. My sense now is that it must be the PCM, but I don't know how to prove it. Also the parts place says they keep the original PROM, they just put it into the new PCM. Please help me out, and tell me what to do. If you do, and I understand that you get tons of questions, how will I know that you've responded? Thank-you for taking the time to read this, it's good to know that there are other people helping strangers. Chip from the TUMC Car Repair Ministry.
eMail: ut4jc@yahoo.com