Check engine light

When my check engine light comes on in a 2001hyndia elantra, when u come to a stop the automatic transmission wont downsift so when u take off its still in drive not overdrive only use this on the interstate or speeds above 55mph. So when I take off manauly 1st 2nd 3rd it shifts then same thing when at a stop but when I turn the car off check engine light goes off car does fine until next time check engine light comes on. Check with my scanner it shows no codes. Try to scan when lights off its like scanner losses communication, I have an autoXray 6000 it doesn't do this with other cars.I haven't changed fliud and spin on filter yet but looks a little dark. Car has 135,000 on it lots of updates have been done this car I just got it for my daughter.