Automotive Forums  Where Car Enthusiasts Go to Discuss Cars Online

My daughter just bought a 1998 mercury sable for 500.00.  The previous owner had just put a "good" junkyard tranny in himself and I drove it around the block and everything seemed fine. After 2 days of driving fine it now won't shift out of 1st gear. The motor idles and runs fine but it won't go over 40 mph and the rpm's go way up. The check engine light is on. The fluid is full. Can autozone check ODBII or do I have to have a dealer check it ?   I'm gonna buy a book tomorrow.  Does anyone have any ideas of what may be wrong ?  Thanks for any help.