This is going to ultimately come down to:
1. Should I replace the upstream O2 sensors, and if so, is there a "how to" video or instructions?
2. Could the problem still be the camshaft position sensor (CPS)? Did I get a bad one that is somehow causing the codes even though the car runs OK?
My car began not starting after driving somewhere. When cold it would start and run fine until I stopped somewhere, then when I returned it would crank and crank but no firing. Then after waiting a few minutes longer, it would start like nothing was wrong. After a few more weeks, it started misfiring while in motion. There was no SES light so I didn't check for codes. I (incorrectly) guessed it was the fuel pump and replaced it with an aftermarket unit, only to see the problem return. I then used an OBDII scan tool and found that the CPS wasn't always working (I forget the code). I replaced the CPS (aftermarket) and it seemed to resolve that issue, HOWEVER then it started throwing SES codes pointing to the downstream O2 sensors. I thought the aftermarket fuel pump I installed was maybe producing too much pressure, which I read could cause this, so I replaced the fuel pump again, and when that didn't fix it I replaced it yet again with a used OEM pump, but the codes persisted. I replaced both downstream O2 sensors and am now getting codes P2a00 and P2a03, which I understand point to the upstream O2 sensors.
I'm not against replacing the upstream sensors, but is that likely to be the true cause? Doesn't it seem strange that both downstream, and then both upstream, would report a problem at the same time?
And, how difficult is it to replace the upstream sensors?