Can an 02 sensor be bad or out of spec but not enough to trigger the CEL? I had a trustworthy mechanic tell me that I need a new o2, but mine is only a year and a half old (Denso from sparkplugs.com) and there is no code thrown. Worth buying a replacement yet?

 

Any sensor can go bad no matter how new it is, but if your 02 sensor is a Denso replacement then i doubt that its bad this soon, but like i said anything is possible.

 

How did the mech come to the conclusion the O2 is bad?

All it does is tell the ECU the mix is rich or lean- not how rich or lean. When the O2 says lean, the ECU takes the mix rich until the O2 says it's rich then goes lean until the O2 says lean... ad nauseum.

 

if you have a problem somewhere else that is causing you to run too rich it can cover the sensor in soot causing it to react slower. you can pull it out and see what it looks like.
it probably wont throw a code until it is completely gone.

 

I believe I have a bad TPS and probably a bad water temp sensor (neither throwing codes but all symptoms point to them), I'm about to replace those but am wondering if I should grab an O2 as well.