2003 Chevy Cavalier VSS or TRANSMISSION SLIPPING

Transmission problem
2003 Chevy Cavalier 4 cyl Front Wheel Drive Automatic

Hi I have a 2003 Cavalier with 96,000 km and bought it brand new with only one major previous problem the fuel pump was replaced, with the exception of last night when the transmission would not change from 1st to second gear. There was also a check engine light on for approx. 3 weeks now, don't know if it's related but I disconnected the battery which reset the check engine light, drove for about 100 feet and the transmission responded and shifted normally but then started slipping again. Could anyone tell me if this sounds like a mechanical problem or could it be related to a sensor for the input or speed control which would prevent the transmission from engaging in the gears accordingly to the speed?