03 running bad when hot

Hi everyone! I have an 03 alero 4cyl that has started misfiring or what it seems like, but only after it gets good & warmed up. I can drive after it's been sitting & it's fine, but drive about 15 or 20 min & the service engine light flashes & the trac off light comes on. When this happens it seems like it's hitting on 2 cyl & it's really tough to get it moving(very little power). Autozone said it was a misfire code & oxygen sensor. I took it to a shop & they told me they would have to replace the cat (340 bucks) before they could diagnose anything else. So I took it to a muffler shop & they said the cat was fine. I've changed plugs & am going to replace the fuel filter. I just don't understand why it only does this when it gets hot. Does anyone think this might be a coil pack or ignition module problem. Thanks for your time!

Just an idea, but my first opinion would be spark problem, maybe the coil pack is bad. I think that if it is bad, it will start to act up when warmed up. This is from info gathered from this site. Again just something to check into. Maybe do a search for coil pack, or coil and see if you find anything.

Spark plug wires can also cause this problem as well as a bad coil module, if it is one particular cylinder. Easy test is let engine run when dark outside and see if wire is arcing against metal somewhere, or you can test wires with an ohm meter. Today's cat convertors rarely go bad if an engine is well maintained, so I would stay away from the shop that wanted to change it.

Thanks for the replies. I'm not sure though that I could see a spark since this car doesn't have wires, but boots that connect spark plugs to coils which are enclosed underneath a plastic housing. And I don't know how to check boots with an ohm meter. If it were the boots wouldn't it malfunction no matter what temp? Thanks again.

Just wanted to let everyone know that the car is now fixed as far as the terrible running is concerned, it was the ignition module/coil which is one piece. We replaced it & it's been running great. Thanks to everyone.

Yep. Same issue I just delt with. Same fix. Ignition control module. I had posted a thread about it a couple of days ago. Updated it tonight with the fix. Apparently the 2.2l engines go through these often. Dealership only had one left. All other parts stores do not stock them and have to order them. Tried every autozone, napa, and oriely's and none of them including the dealership can test this particular module. This car had the problem full time though. Not just when hot. Hitting on two cylinders and dumping gas in the exhaust causing a cat failure.