We inherited a 2003 Deville from my fathers estate. It was always maintaied at the dealership and has only 30,000 miles. It ran beautiful until one day it started missing and the check engine light came on. I was involved in an accident and the front drivers side fender, grill, etc. was damaged. It was painted. We had it diagnosed and it was misfiring on plug #5. Our mechanic pulled it and said it was ok. We took it to the dealership here in town, and they diagnosed it as both coil packs needing replaced. They said they would need to do a repeat $170. diagnostic on it after the $917. repair to be sure there isn't something else wrong. We switched to 5w-30 synthytic oil about a week or so before the misfire started. Also we were told that the ethanol gas mixture could be causing a problem. The tech at the dealership said it was an environmental issue, whatever that means! The only thing I can think of is the body shop using heat lamps with the hood up to dry the paint. Could this be a possible cause for the coil packs to malfunction? Could there be something else causing the misfire?

Keep driving it with the misfires and you'll soon be replacing the CAT convertors for BIG money.

It's parked.

Somethings very, very fishy about needing two coil cassettes at 30,000. (Even 130,000). The parts, depending on whether they're ACDelco or Airtex go for $220 or $190 apiece, so that's $400 and an hour's labor. The dealer is charging max on everything.

The problem could be caused by something as simple as moisture or carbon tracking in a coil connector boot: these engines have no plug wires. Each of 8 coils connects directly to a plug with an insulated connector boot. I'm not saying the cassette for the #5 plug side isn't bad, just unlikely.

It certainly ain't caused by ethanol, heat lamps or synthetic oil. Pull the DTC's yourself. See the sticky in the Tech Tips section.

What is telling you it's misfiring?
If only that one plug is misfiring, swap the coil packs around and see if the misfire plug moves.
Easy way to tell. Is this the 4 coil pack ignition or the one that has the COP (coil on plug ) things?

Foos, 2000+ is COP ignition. He has an '03.