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?

IIRC, it was a bit hard to get too...I didn't have the tools and I do most of my own work. I only got one side done, barely. There is a HowTo in my signature for Oxygen Sensors if I recall correctly.

Ilya wrote: ↑

Fri May 08, 2020 11:55 am

IIRC, it was a bit hard to get too...I didn't have the tools and I do most of my own work. I only got one side done, barely. There is a HowTo in my signature for Oxygen Sensors if I recall correctly.

Thank you for the reply. The How-To guides that you link to include downstream, but not upstream.

Not to muddy the waters any more than they are, but sometimes codes are not the sure thing that they appear to be. I had an Escalade that registered a code regarding engine trouble. I don't remember which specific part was flagged. The dealer thought the car was way too new to throw that code so he did an in depth search with his fancy dealer scanner. He decided on a hunch that I might have a ground issue. If I had replaced the part that coincided with the code, it would not have fixed the underlying cause and the computer would simply show the next part down the line from the faulty ground. I probably made a hash of that story because is was years ago, but the point is that the dealer had a way of looking at every detail in real time and surmised it could be a ground fault. After he investigated that hunch, he fixed that ground fault and all the codes disappeared.
Maybe your dealer, if you can trust them, may be able to look deeper and consider other things that can cause that code.

I'm thinking of replacing the camshaft position sensor again, since that is when the O2 codes first appeared. It's also a lot easier to replace than the upstream O2 sensors. I'm planning to sell the car soon, I managed to get it smogged between resetting the codes and them tripping again, and it's not worth enough to spend $$$ getting it diagnosed and fixed. If the CPS doesn't fix it, and either the upstream O2 sensors don't fix it or they're too hard for me to do, then I'll just sell it as-is, smogged for the next two years, running well, but with SES light and codes.

If it is an M45, my advice for the crankshaft/camshaft sensor issues is to always go with the genuine OEM part. It might seem more expensive upfront compared to aftermarket units, but always comes out cheaper in the long run.
That's where I would start.


If it comes to it: I found the upstream sensors quite accessible with a proper o2 sensor socket & long extension + ratchet.