Rebuilt 4l60E delayed first shift

Trans stays in first gear too long. It shifts at 3500 and then all the next shifts are normal. As I understand the solenoids are both on for first and then 2 is turned off to get second. This is an 02 Avalanche. My rebuilder says it is electrical but I don't know computers only manual valve bodies. It seems that it is a spool sticking or valve body not torqued correctly. Anybody have ideas? Thanks Billy

There are lots of possibilities but I would say the first thing you need to do is have the truck put on a scanner and wacth the gear command, The will tell you wether the issues with the truck or the trans

What type of scanner is required,more than a code reader? Should I go to a transmission shop rather than a local shop? Thanks Billy