This sucks. My car is intermitently giving me a code 42 after about half an hour of driving. It runs perfectly fine until it sets. When it does, it lacks power, then about 3 seconds later, it will set the code and trip the ses light. I can cut the ignition with the car driving, then flip it on again, and car will run good for about another 5 minutes, then set the code again. I've checked the wiring for shorts and grounds from the ignition module to the ecm and it checked fine. I attempted to rebuild the stock dist, but when i tried to take it apart, it was so fragile, it fell apart. So I decided it was time to upgrade and I bought a new msd billet dist. I don't know what else to check for unless the ecm is bad.
Here's the part that really sucks, I just got my car driveable after 6 months of rebuilding and restoring. I got caught up in a flash flood, and long story short, my car was underwater from the top of the headlights in the front to the center of the rear wheels in the back. The first thing I did was grab the ecm and get it to higher ground. I got it towed back and immediately changed all the fluids, and cleared it out. I had it running again within hours of getting pulled out of the water. Since then i put in new carpet, 4th gen seats with new covers, and built my own stainless exhaust. The water was butt level inside the car and was just shy of my radio and the wiring under the dash. I knew i would have problems with my car, since the flood, but this intermitent problem really sucks.
Anyone have any ideas?

 

This is a flow chart to help you diagnose your problem. Hope this helps.
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Well, that did not work! Here is a link to a site that I use to diagnose SES problems.http://chevythunder.com/est_code_42.htm