Hi, I have a 2004 Infinity g37x and recently had an air fuel ratio sensor replaced but the mil light keeps coming back on. I've taken it back 2x to the garage to check if they did the relearn and they said they did ..they think the code is now for the downstream sensor. My question is..will the upstream sensor and the downstream sensor be the same code? P1273 is the code I keep getting.

P1273 is upstream. The rear O2 is strictly for the government and has its own set of codes. There's a bulletin for P1273/P1274/P1283/P1284, older Infiniti ECM firmware has a glitch that causes those codes to keep recurring unless you clear out the self-learning values. Your repair guys probably didn't know about it. Here's a link to the bulletin:
https://www.autocodes.com/uploads/infin ... 04-059.pdf

Thank you.....I had a copy of that bulletin when I went back to the garage the first time..then looked at it and gave it back. And when I got my car back 20 min later the light was off but soon came back on..so I called the garage again and they looked at it and that's when they said they think it's the downstream sensor this time. So I took it to a Nissan garage and waited an hour and a half just so they could give me some crap about the cat coming apart (which I had checked out and it's fine).
So now I'm not sure what to do..

You'd be getting downstream codes if it was a malfunctioning cat. That's the downstream sensor's whole mission in life, to tell you if a cat is going bad. We techs have a saying about it, "The front sensors are for the ECM, the rears are for the EPA." The fronts are mixture detectors and they're essential, the ECM needs them to run the engine. The rears are metaphoric tits on a bull, take them away and the car can still run fine.

However, that's not to say the cat can't be responsible for your issue. It just won't be because it's malfunctioning. P1273 is a lean code that usually isn't from a vacuum leak or bad fuel, instead it usually indicates something further down the system is leaking. So partially plug your exhaust pipe and check for cracks, pinholes or bad gaskets on your bank1 manifold and cat. That's the next most likely culprit.

I'll check that out ....thank you for the help...much appreciated