Engine won't start

[hezzy]

Hey guys, I am new to the forum. I love supras... yes. I have a 1987 supra non turbo of which i have done a ton of work on. Dropped in a new engine and all.. all original. New engine has 35000 miles on it and just recently it won't start. I pulled off some codes: 12 and 21. 12 is a rpm problem while 21 is o 2 sensors. But the car won't even try to start. I also replaced the starter a while back... the only thing i can think of is the computer is killing the engine. Any ideas... thanks soo much in advanced