in my book a code 42 is: 1) Electronic spark timing (EST) circuit - open or shorted; 2) Direct ignition system (DIS) fault - bypass circuit open or shorted to ground during engine run; 3) Fuel cutoff relay circuit - open or shorted to ground
I am not a mechanic but that is straight out of the code book - if anyone can help him figure out how to check these things, I am sure he would be greatly appreciated
FYI, I had a similar situation but mine was fixed with replacing the ICM, cps, coil packs, plug wires and plugs and that was all - I have the 2.5L 4cyl in a '90 lumina that now runs great at a little under 220,000 miles with no oil leaks, no oil burning and no coolant leaks - sounds like yours is more than mine - hope this information helps a little