I have a 2003 A4 3.0 with 151,000 miles. The engine light has been on for some time and the diagnosis codes are coming up with oxygen sensors. I know this can be the beginning of a long path in trying to correct the "engine light" so I thought I'd reach out. They are the original sensors so I'm sure it wouldn't hurt to change them. Any suggestions on part numbers and auto parts stores where to purchase (Ex; parts geek, Pelicanparts) and should I change just the upstream or downstream ones first? Also my check oil light is on (and I don't need oil) which seems to be a issue with this model as well. I know there is an oil sensor on the bottom of the car that can be changed during an oil change, however is there another one of the top of the engine?