when i start my1996 gmc sonoma it revs up to about 4500 rpm amd will not come down to idle. unable to get codes at this time

Vacume leak?
Stuck gas linkage?
What engine?
Any check engine lightss?

What all have you done to possibly remedy this problem? I would guess stuck throttle. Check your throttle cable, cruise cable, gas pedal, and throttle body (with engine off) and make sure they all move freely and are free of any obstuctions. Post what you find out and we'll move on from there. Good luck.