MLHG

If the code says i need to replace 1 O2 sensor, should i just replace all of them? If the have to pull the engine out.

C5 A6 2.7T 6sp

the green amigo

I'm no expert but there is no way the engine needs to be pulled to change O2 sensors. The job is a PITA because of limited access but can be done with patience.. and small hands help. Everything is accessible from the top with removal of only a few accessories.

As for changing them all, I would definitely change both upstream sensors. The other two downstream didn't throw any codes. That's what I did and they worked fine afterwards. Hope that helps.

nyc.turbo

You should monitor sensors activity to see if its really bad . Any way why change 2 sensors if only 1 is bad