JCZVM

Anybody know why my Z goes 6/7k RPM while im on highway 60mph it sounds like im putting too much pressure on the Z before it wasnt doing that. it goes to 1st gear then 2nd but on the 3rd it just keeps on going on the RPM it wont automatically switch to 4th gear .people told me it might be on limp mode/might be your speed sensor/catalytic converter?
took it to the dealership they said i had to replace the passenger side catalytic converter.
My friend plugged it up to his error code device and it output speed sensor popped up.
If anyone can help me out with this. thanks