I did a complete rebuild on a seized 99 silverado 2wd. 4.8L vortec with the 4L60-E. I put in a 5.3L Vortec, and used the same tran. the tran pump was pooched so I replaced it, but now the truck still wont move.. gear select linkage was replaced because it was falling back in gears. the truck is automatic obviously, I am just wondering if the torque converter could be pooched after the truck seized? (it wasn't an immediate seize) is there possibly a security program in the 4.8L's computer that wont let the truck move? because whenever the truck is running the instrument panel has a yellow flashing light saying (SECURITY). I am new to chevy's so I am clueless. I did not replace the computer or get it reprogrammed but the engine runs. I hooked up the OBD and I had mutiple random misfire twice, and something with the MAP sensor.. any ideas will be greatly appreciated..

 

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is there possibly a clip i could've missed or something that would cause the tran to not move other than the torque converter?