Thread: Running a couple codes. Mystery solvers take a look.

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?

I have a 2004 with the same p0420 code and bad hesitation. My plan was to buy a 3" catted downpipe from zz performance and hope that solves the problem plus get rid of the u bend in the factory pipe. As for the plugs I used the AC Delcos before on another 2004 that used to own and didn't notice any diffrence. From the research that I have done everybody suggests running autolite plugs.


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The check engine light will flash sometimes when I floor it yes. Here's a quick update, too.

I decided to pull out plug 5 and take a look at it. Calcium. All around spark plug change. I just decided I was going to use the Bosch ones for now and see how they do. Put the new plug in 5 and is was great. I go to take out 3, and I can't get the wire off. I pull a little harder, and it won't come. I pull really hard and the wire breaks. The collar comes up so I can see the metal on metal connection between the plug and the wire. At this point I say to myself "Well, looks like you're doing wires today too." Only way I could get it off was to break the wire completely so I could get my socket over the plug and the connection from the wire that was still on the plug I had to cut off. I get it off, and get the new one back in and put the broken wire, which is still attached on the other end to the coil pack, right next to the plug so I know which one is which when I replace them tomorrow. Move on to 1 and have the same issue. Now I can't get plug 2 off, especially since they decided to put the back plugs in the absolute least convenient spots, and I can't get the leverage to pull the plug off, let alone try to break the cable free like I had to on the other ones. I'm still stuck with the hood open trying to get the back plugs out. Needed a break so I came back upstairs to the computer.

Autolite wires, by the way. Immediately a bad name in my book.

It took me almost half an hour on each 3 and 1. 2 is not coming off for anything.

I usually use a pair of pliers to get ahold of the boot and pop them off.


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Oh, I was grabbing by the boot. Always. I'll show you what I've got, because the dude I bought it from was not using OEM parts. He bought wires with this metal sheath around the boot, so not only is it a ***** to get off, it's almost impossible to grab more than a half inch of the rubber boot itself. It's almost like it's a locking mechanism that doesn't let you remove the wire once it's on. Weird thing is that 5 came off super easy just like regular wires.

I've never seen these on other cars I've done plugs and wires in. That's odd. Alright, I guess. The things still aren't coming though. Just gunna have to muscle them off. Thanks.

Second thought: That white part on the spark plug is made out of ceramic, right? Why not just grab deep with some channel locks, break the plug in half, and get my plug socket in and get it out that way?

Update: Finally got the wires off, got the new ones put on. While I was doing this I had my battery unplugged I don't have an OBDII scanner, so I just let the battery die and jumped it when I was done to reset the engine light. Started up the car, drove for a good 25 miles and no light. I also noticed the hesitation was completely gone. I'm going to finish out the week and see if it doesn't come back on. If it does, I'll be back on here.

Here's my thinking now... Maybe the misfiring plug or wire was causing the catalytic code because it wasn't burning the fuel in that cylinder and that's what was making the p0420 code come up. And the gas cap code didn't go away because I didn't reset the computer after I bought the new cap.

How likely is this scenario? Because I would totally be cool with not spending 300 bucks on a new cat.