O2 sensor, hmm. See - I would have thought that any failure of the exhaust would be considered part of the drivetrain.
For an O2 sensor to fail at 51k miles, that makes me frown. My `04 Turbo with 137k miles is still on all original O2 sensors.
EDIT: AND - that would make sense if it was surging/hesitating with no CEL - but now as the sensor gets progressively worse it will eventually throw a CEL. I'm sure it was confusing the hell out of the computer!