I searched to try and find out this info before hand and couldn't find it so sharing for those looking for the info in the future.
I have a 2011 Altima 2.5 SL and I had the typical whining as the major indicator that the CVT transmission was about to go out. Limped home 25 miles then towed it to the dealer. The DTC was P1778 Step Motor Function. The step motor controls the main pulley that controls the tension on the belt to manipulate it up and down the cone, from what I understand. Its part of the valve body inside the transmission and as you may have heard, they don't really rebuild CVT's, just replace. So the call came in today and its $3630.00 to put a new one in. If you find yourself in the similar boat, I feel your pain and $3630.00 is the cost for the dealer to fix your pain. lol. Best of luck!!
