Pontiac Repair: G6 running rough, vvti engine, vacuum leak

Question
My Pontiac 2007 G6 4 cylinder engine is letting 27% oxygen into the system when it is idling. I have replaced the oxygen sensor and now it is getting less fuel mileage then before. It runs rough and when you kick into it it is really sluggish. I need to know what other things I should replace to see if this problem can be fixed? Any help you can give me would be greatly appreciated thank you so very much.

Answer
Hi Sandy:

how do you know that % of oxygen in the exhaust? did this fail some sort of state emission test? is there any service engine soon lamp on on this car? do you know which 4 cylinder engine is in this car? there are 2 versions of it of is a VVTI engine and one is a basic 4 cylinder. If it's running rough enough it should set a trouble code and the engine lamp may be falshing if it truely bad enough. This should still be under warranty as well if it's less then 36K miles. You could have a vacuum leak some where and that would have higher then expected oxygen levels in the exhaust.