The 1638 code has to do with vats, that's why it wouldn't start. it's for a 4.3L so the calibration is not setup for your truck.

simply have it flashed to an 02 truck with the same engine type as yours and then try the passkey, it should work. if not, it's a funky one and you'll need vats turned off.

the 02 pcm will cure your kr problem if it's not a mechanical or sensor problem.

good luck,

allen