Thanks for your extensive posting. I'm sure it's been pain for you. I have 2000 RX 300 with 140k and it started having very similar issues. My trusted mechanic replaced O2 sensor but CEL came right back. After getting some maintenace jobs; new belts, battery, etc., CEL still appeared after 5 to 20 miles of driving. One thing I noticed is that when I drove locally (low speed), CEL came on around 5 miles of driving. When I drove high ways, CEL didn't come on until I slowed down to local driving.
I tried this after resetting CEL, because after reading yours and others' postings, I suspected an issue with fuel injector.
I wanted to try fuel injector cleaner before I try to get a new set of fuel injectors. One review site recommended 'Lucasoil' and 'Techron' brand. I bought Lucasoil at Autozone for $10 and started using it. What a surprise, on the first day, CEL disappeared. It came back on second day and I added more cleaner and on third day, that dreadful CEL disappeared again. It's been only few days and I've driven 50+ miles with highways/local mix. So far so good.
When CEL came on first, it gave me a whole set of code, 0300, 0303, 0171 and others. I believe there were like 6 of them at that time. In any case, I think I was in luck to get CEL out with fuel injector cleaner but for those who don't have much skills in mechanic division like me, it wouldn't hurt to try fuel injector cleaner first to see if that takes care of the problem.
I'm getting ready to tackle that intake valve cleaning thingy for low idling issue but I hope I can help few to save time/money/agony to go thru this CEL problem.
Thanks,
