2003 GMC Sierra 1500 Transmission and OBD2 read error

We recently just changed the transmission oil in our GMC. We accidently overfilled it by a quart, which caused the transmission to not shift into 1st gear or overdrive. So it will only run in reverse, 2nd, or 3rd. So we took out the overfilled oil and it still did it. When we try to read the computer with our Innova 3100 OBD2 reader, it just says error. We've tried resetting the computer by disconnecting the battery. But it still does it. Anyone have an ideas? Thanks!

Kevin