Agree with Frenchy, the factory service manual will prove invaluable (you "might" be able to find a copy online, post back here if you can't).
In the meantime:
12 = Mass air flow sensor circuit
21 = Ignition signal circuit
34 = Knock sensor circuit
51 = Injector circuit
For the 34/knock sensor circuit, if the knock sensor is in fact faulty, I would recommend relocating it to the top of the intake manifold so that future service/diagnostics are easier. (It's buried underneath the intake manifold and is a pain to get to). There are lots of discussions/writeups detailing how to relocate it.