I've had the check engine light come on for a bad tank of gas, not tightening the gas cap down enough, clogged PCV valve, and driving up in altitude.
I recently had my EGR and PCV valves replaced and the hoses going to them cleaned out. Since I had this done I have gotten 20 more miles per tank of gas than I did before the clean. Give that a shot. Not too expensive, but well worth it.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless
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