check engine light - O2 vs Coolant temp
 

What are the chances of a fairly experienced mechanic missing the call on this? Check engine light comes on... Mechanic says O2 sensor is out of spec and replaces it... 3 - 4 weeks later the light is back off and on, then stays on. Gas mileage takes a dive. New mechanic says it's possibly the coolant temp sensor bad and telling the computer the engine temp is cold, hence a consistent richer fuel mixture. After a week or so of the rich mixture, the O2 sensor gives bad readings, showing up in the codes when the check engine light comes on. What are the chances a mechanic just goes for the O2 sensor and doesn't find the coolant sensor problem.... until it all happens again?

I don't see any posts about the coolant temp sensor being a common problem, unlike the numerous ones on the MAF and the harmonic balancer.

Mass AirbFlow sensor

Find out what the code is and the description.
There are FOUR O2 sensors, if this dufus can't nail down what's wrong (Mmmmmmmmm, it's the O2 sensor. NO? That didn't fix it? I think it's the coolant temp sensor now.......) then maybe he also can't figure out which O2 sensor is bad.
Gilly