I bought an '04 grand prix almost 2 years ago now and it's easily the best car I've ever owned and worked on. There's still a few problems, but I thought I could ask and see what you guys thought.

It seems I'm running 3 OBDII codes that I need to get rid of by December 31st (emissions testing) What I have are as follows:

p0420: Had this one for a long time. Low efficiency catalyst system. WHen the light came on for this, I started noticing some performance issues with hesitating, and feeling kind of like a misfiring plug, but not quite as severe. It got worse, and it hesitates pretty bad when I really get on the accelerator. I've been told this is the code for when the post-cat o2 sensor registers a bad read, so I've come to the conclusion that my o2 sensor is fouled, OR I really do need to replace my catalytic converter. I've bought 2 bottles of "Cataclean" from AutoZone, and it did help that hesitation a bit, but the hesitation is still there, especially at higher speeds. It's almost like it goes into a higher gear immediately after stepping on the pedal, since it it'll just throw my RPM needle down and the engine sounds like it's just trying to catch up to itself.

p0455: Evaporative emission control system leak detected. This code is essentially the "Check your gas cap" code. The guy I bought the car from bought a locking gas cap. I figured it was just old, and he had bought a cheap one. I bought a new gas cap (which also locks). The HUD in my car is also telling me to check my gas cap.

p0305: Cylinder 5 misfire.

My thoughts: It seems to me that all of these codes could be coming from one faulty part. A bad catalytic converter or some other part of the emissions process or something. Like I said before, I used a couple bottles of Cataclean with moderate, but not ideal results. I replaced my gas cap, and just for the hell of it I bought new spark plugs, because I figure it's probably time for those anyway. Bosch Platinum-Iridium Fusion. I have not installed these yet because after I bought them, I've been reading that multiple electrodes can cause misfires (there's 4 electrodes on each of these). I've also been reading that AC Delco Iridium plugs are the best way to go.

Congrats for reading this long, You have the background information. Here comes the questions I have for you all:

Is this something that anyone else has been having a problem with and if so, what was your fix?

If I have to pick up a new CAT, would I be fine getting one of the magnaflow universal cats, or should I try to spring for the direct fit options?

What should I do with these spark plugs? Install the Bosch, or take them back and get AC Delco Iridium?