Please Help! I Am Going On A Long Distance Trip!

I have a couple questions. Which I am sure isn't out of the ordinary here...

I have a '97 Grand Prix GT.. 3.8L V6.

I noticed some time ago that my dual exhaust is shifted to the right and is vibrating up against my bumper or (rear ground effects). Do I need a new bracket to be installed? And if so, how much of a repair cost is it?



And number 2... I noticed that my check engine light keeps going on and off, some say it is due to the weather change here, but when I had it tested it said my O2 sensor was bad. If that is so, why would the check engine light now be off? Is it still bad? Also.... would that be the reason I am getting such crappy gas mileage right now?
What else can I do to make my gas mileage better for a long trip? I am going on a 600 mile trip here in a few weeks...


ANY help would be appreciated!!!
Thanks!

there are rubber hangers on the rear exhaust, and when one breaks, the exhaust will shift toward the side that still has the hanger.

An oxygen sensor can intermitently short out, so it's not out of the ordinary for the light to go on and off. If the CEL is flashing, that indicates a misfire condition, usually caused by worn out spark plugs and wires.

On your 97 GT, there is also an issue with the upper intake plenum ( it affects all GM 3800 Vin-K engines from 95-99, and many after a revision in late 99 all the way to 04) that will cause bad mileage and the CEL to illuminate. If you have been using a little coolant, the UIM may be damaged and leaking either into the engine, or from under the throttle body outside, due to an EGR stovepipe issue that warps the UIM.