Mazda Repair: Mazda 626 Engine Light, oxygen sensor, mazda 626

Question
My engine is 4 cylinder and my car is a 1998.  So is my problem a oxygen sensor?
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-----Question-----
My engine light comes on only when the weather starts to warm up.  Usually it has to be 40 to 45 degrees outside to have it come on.  I had my mechanic run a diagnositc for the code but it did not come up with anything.  Is this problem a oxygen sensor?  The reason I ask is that sometimes my car seems to struggle with accelteration and with running smoothly.
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never said what year it was or what engine!

Answer
cant say it is a oxygen sensor, but from what you said about running smoothly i would think not, i cant understand why 'you mechanic' cant retrive a code, when the check engine light comes on it always sets a code in the pcm. i want you to try this, get a can of brakeclean or carb clean and while the engine is idling, spray it along the intake gasket (back of the engine)  see if the idle starts doing weird stuff, try that first then let me know