I have a 2000 A6 2.7T that won't start. I purchased a Ross-Tech VAG-COM to see what codes it was throwing but I only get the same 2 codes that were showing when I bought the car (& it ran fine then). The codes are 16486-MAF, 17521-O2 sensor. Physically it sounded like a fuel issue because the car would crank over then the motor would shut off. A few hours later the car started fine. The next day it wouldn't start. Then 2 days later it started fine, then hasn't started since.
I'm confused because I'm not getting any other any other codes like fuelpump or relay, but I didn't think a MAF or O2 problem would cause the car to not start, especially intermittently.
I just don't really want to spend the money on the expensive *** O2 sensor if it's not gonna fix the problem. At least the fuel pump relay is only about $40 at the dealership.

Any suggestions would be greatly appreciated.

 

I don't think you'll see a code for fuel pump or relay. A quick check for fuel pressure is to loosen the fuel hose at the union just above the airbox, have a friend turn the engine over and expect a bit of a spray. Very dangerous if you are not careful! Use plenty of rags to soak up the gas. If no spray, your pump is not pumping (no ****!).

Your O2 sensor would not cause no start or no run condition - you could disconnect them if you wanted to rule them out, the engine would run badly but would not cut out because of it.

My money is on what you have guessed - no fuel pressure.

 

When you do the fuel pressure check, put your thumb over the end of the hose. If you can hold your thumb against the pressure, the pump is bad.

Bob