So recently my 4th gear is slipping when I'm on auto, but seems to be fine when I go manual. No noise whatsoever. My Ralliart just goes from 1,2,3 and redlines and skips 4th. Had my mechanic look at it he got code p1879-4th gear block.
-58k miles on line
-just had the transmission fluid change 2months ago
-daily driving and don't race it
Can anyone help me figure out what causes this and know how to fix it or share a link? My warranty just expired and any help would be much appreciated. Ty!

 

If it were mine, I'd start from the possible correlation with the fluid change and the problem occurring. There are really only two things to check with that:

1. What fluid was put in it? Was it factory Dia-Queen SSTF?

2. What quantity was put in? Was it the correct amount as per the service manuals? Think that's 5.3L (for a straight "drain and refill", without any filter removal/replacement).