transmission shifts hard

i have a 2000 grand am with a 3400 engine and stock trans, i did a full engine swap in the car a few months ago for a friend and it was running fine, the engine and trans came out of an 04 grand am. i bought the car for a second vehicle for my every day driver after he changed the (tfp) sensor, and it shifts fine, but it is still showing a P1810 code, and shifts hard after driving it for a while but only when putting it in gear. any idea on what it can be?

Welcome to AF cameroman70 !

It has to do with the years being different newer models, 2003 and up dont work well with prior years without modifications. Here is a link from HP tuners, i dont usually share other forums but is a great write up on this issue!

http://www.hptuners.com/forum/showthread.php?25311-dtc-p1810

my 99 SE have a 3 valve VB, but its been highly modified and rewired. not much fun there! lol

Hope it helps!