Hello guys!New here sorry if this is the wrong place to post but I’m having problems with my Transmission it seems like it’s starting in 2nd or 3rd My guess is 2nd because it still shifts when starting to gain speed but dosent go into OD I believe could be wrong.but I have no first gear I can’t
manually go into it either and in reverse I have to go up to 2500 rpm to start moving a little as well with any other gear I have to rev high to start moving.Any help would be appreciated kind of stumped here I tried looking at the diagnostic chart for both these codes but pictures are too blurry.background on the engine it was swapped by previous owner from a 4.3 to 5.3 could this be a internal problem?I wanted to ask on here before I go waste money on a new transmission and torque converter.

What I’ve tried:
Replaced internal transmission harness and all 6 sensors as well as the manifold pressure switch.I tested the external harness wire E for 12V and in fact did get 12 volts,I replaced the filter new oil I’ve checked all the engine bay and inside fuses they all checked out.I did not check the relays below is a picture of all the sensors installed

 

Welcome to the forum, first question is had the transmission been working correctly after the engine swap? The P0748 can be caused by a amperage drop to transmission. Most times it’s while starting the drop occurs with a failing ignition switch. Is the Neutral safety switch working and adjusted properly? Disconnect the case connector at transmission and then start engine put in reverse And see if the reverse issue has changed.
Brad