By low gear, you mean the auto shifter moves to L position by itself while driving? Or does the auto transmission shift to lower gears causing higher engine RPM on highway?
If the car downshifts automatically at high speed, it could be due to bad transmission fluid, or the transmission itself could be failing.
Watch this video to see how to check transmission fluid on Ford Fusion: (start from 2:15)
Watch this video to see how to change transmission fluid on Ford Fusion:
