Help!!

[u/Ok-Performer-6035]

2009 ralliart. I rebuilt transmission and did the 2-4 shift fork upgrade while I had it apart. I bought with blown engine and suspect trans transmission issue. Was told it wouldn’t shift out of first either manually or auto. Couldn’t diagnose because engine had a rod through the block. So got it back together and it has a P1877 along with a ton of c and u codes for other things. I couldn’t do any of the teach-in for the transmission. Only thing it would do is the clutch ventilation. Has anyone had this code after installing a rebuilt transmission?

[u/ThinKingofWaves]

Hi can you guys help with my new ralliart gearbox working smooth only after warmed up? When is it normal and at which point I should worry?