I have a 74 c10 5.3/4l60e swap. I have no issues with the swap and can drive it anywhere. Problem is my converter locks up in 3rd. When I floor it the truck stutters n hesitates a few split seconds then downshifts WOT. I had it looked over by the tuner and he showed me on the laptop it shouldn't be doing it because he has it to lock in 4th. He manually unlocked it and man it was night n day. Now mind you I do a p0121 tps code but I had that code come up once before and it hasn't done that. I have edelbrock pro-flo xt intake and matching 90mm tb. I want to know does the p0121 have a direct link to the converter locking up in 3rd or is it something in the transmission?

 

I would say that if your Tuner manually unlocked the converter with his scanner then its not a trans problem but a tune problem.