Code 42 is the biggest horror story on my 87 4-cyl. Fiero with DIS.
I didn't know my Fiero that well when this happened some years ago. All of a sudden, the Service Engine Soon light was turned on, and the car started running miserably. I drove it a few miles to a dealer. They said it was just a loose vacuum connection and charged me $180 for fixing it. Those guys didn't even bother to check what the code meant!
The second time this happened a couple of days later. The car ran just fine until the light was switched on, and quite poorly after that. Once again, I limped to the dealer. Now I noticed that the exhaust manifold was red hot, and the insulation and tie clips on the wiring harness at the firewall had begun to melt. The problem started to look serious. This time the dealer checked the trouble code, and decided I need a new ignition module. The module checked OK according to the shop manual procedure, though, so I was wondering whether that assumption was correct. They insisted that had to be done, but of course they didn't have the modules in stock. Well, I ordered one from the States, and it was express delivered in a couple of days. It cost me just a little over $200 plus $100 in postage and taxes, and some $400 for the debugging and labour at the dealer. Result: a couple of days later the same problem reappeared.
This time the dealer said it was the ECM. I ordered a rebuilt ECM from the States. It was express delivered again, $150 + core charge + something like $100 in postage and taxes. The ECM was replaced by the dealer for a mere $500 or something. I was starting to get pissed.
In the meantime I had written to FOCOA for help. Bill sent me a copy of Pontiac's service bulletin that said that code 42 can also be caused by electromagnetic interference from the spark plug wires getting coupled to the signal wire harness that runs into the DIS ignition module, if these wires are routed too close to each other. I checked mine, and the spark plug wires were practically lying on the signal wires. The car had been at the dealer three times, and they had not checked for this! I rerouted the wires as far apart as I could. I don't know if it was necessary to replace the ignition module or the ECM, but I didn't want to reinstall the old parts to check, and risk a total meltdown of the car at some distant highway. Anyway, after rerouting the wires the problem has not reoccurred.
So, check the routing of the spark plug wires before going through any high cost component replacement. That may be all it takes! Also, be sure that the spark plug wires you have are of the correct type. They need to have a fairly high resistance for EMI suppression. It is also possible that the coil(s) have a temperature sensitive short or bad connection, so you should check them again at different temperatures.
From: Alve Jukka