99 Sunfire Transmission

My son has a 1999 Sunfire (automatic). It starts off great and gets to about 40 mph. Then it seems as though it won't change gears. It just stays in what appears to be first gear. Does anybody have a clue what this might be?

A buddy of my suggested to disconnect the battery so that the computer would reset. We did and nothing happened. By the way we just had the transmisson serviced. Strangely we had to add 3 quarts of transmission fluid but there are no leaks anywhere. LOL

We're hoping it's something simple. Please give feedback to this overly concerned dad.

I have a 99 sunfire also. i had the same exact problem your having. i took it to a pontiac dealership and got the TRANSMISSION FLUSHED(CLEANED) and now my car is running fine. i dont know if you've tryed this but its probably the best thing to do for your tranny.