Problem with 99 Grand am

[Justaboi1981]

Here about 2 months ago. I had my car taken into the shop due to a check engine light that was on. I found it out it was due to the fact a sensor behind my cadlict converter had gone back. Also he replaced the spark plugs due to them being having large gaps in them and also them being bad due to a sensor making the car ride rough. Well since then the check engine light as gone off but not. When I start the car cold. I get a hestation. If i open up the engine by giving it some gas it usally goes away in a matter of seconds. But it happens every time the car is setting a while. Also it seems to be worse if the gas tank is under 1/2 or 1/4. Also since them my glass mileage has gone WAY down. I was getting 280 miles to tank maybe 300 if i let the light stay on a while. Now its like 200 - 220. This has all happened since me taking it into the shop. Could anyone please give me an idea of what might be wrong.. I don't see any warning lights?

[nova_gh]

i'd maybe start with the spark plugs. I just replaced my spark plugs, after that my car had hesitation on accelarting around certain speeds. Double checked them and put it back together and its ran fine since. Reinstalling the cover thing for the spark plugs is a pain. Your mechanic may not have gotten it tightened down right and theres a loose connection. Also, the spark plug gap should be .060", looking at that at first i thought that was way to much gap, but turns out thats right, maybe your mechanic thought that was too much too and reduced the gap size??? Also try to look for a broken vacuum hose, those things can cause all kinds of problems.

[DoctorSpeed.com]

My guess is a bad 02 sensor.