I was able to track down the dealership that, indeed, changed the timing belt (at 76k miles). Phew. That saved me $$$ But, My car is throwing two codes;

bad camshaft position
bad engine temp

I have confidence that those are actually the result of a bad O2 sensor. All the other symptoms of a bad O2 sensor are there; poor gas millage, rough idles, engine sounds like it's working too hard.

Is this a cheap fix? How do I know which 02 sensor is bad? Please help!