[pcmos]

Have you checked to make sure you aren't leaking coolant into your engine through the intake manifold? You should check your coolant level, re-fill if low, and if it drops again with no visible external leaks then you've probably got a blown intake. That would result in a fairly significant drop in gas milage and could avoid setting off an SES. As for the transmission over-fill, I made the same mistake after replacing my transmission filter on my 2000. Having it over-filled dropped my gas milage by about 2 or 3 mpg. I had to go purchase a hand pump at pep boys to suck the fluid back out through the filler tube, as I did not want to break the pan seal again. I could also feel a difference in how the transmission behaved with it over-filled, it felt like it was constantly straining, engine temperature was consistently higher also. It would probably be a good idea to simply track down your transmission fluid leak and fix it.