2001 Buick LeSabre Q&A

2001 Buick LeSabre Question: Transmission slipping on hard pull

I have noticed my transmission slips when I go uphill. I have noticed that the transmission is not down shifting and then it will begin to slip. Also my gas mileage has dropped from 24-25 mpg to 20-21 mpg. I have had the codes run on car but nothing comes up. -
Answer 1
Just go down hills -