Grand Prix GTP 2 Codes thrown.....

My GTP has developed a stutter when you start pulling a good grade, just as the boost meter starts to register. A downshift or hard acceleration will bring the car to life and appearant smooth running. The SES light is on. 2 codes have been thrown P0301 - Misfire Detection in cylender #4 and P0420 - Catalyst System Efficiency Below Threshold Bank 1. I figure that 1 problem is causing the other. The problem being: is the misfire causing the other code to register or is the CAT or Oxygen sensor causing the misfire? Any help would be greatly appreciated? Whish one is more likely to cause the stutter when the super charger starts to produce boost?

Thanks

I have P0420, but its because i'm running a high flow cat, yeah I even have an o2 extension harness, the code is still there, and with the '04+s if you run a high flow cat, it seems to be hit and miss, some guys can pull it off and not throw a code, i wasn't that fortunate...

P0301 is misfire cylinder 1, not 4 (that may have been a typo, but we want to be clear). It's possible that if it's a spark induced misfire, the unburned fuel entering the exhaust is tripping the other code. The stutter is caused by the misfire, no doubt. If it's been a while since you tuned her up (or if you installed junk Bosch plugs), put a set of quality plugs and wires, pcv and fuel filter in and all your problems could be solved.

As suggested unless already done start with new spark plugs and ignition wires.

Note that the P0420 DTC is not dependent upon misfires being the cause. It means the oxygen storage capability of the CAT convertor is less than desirable and it has detected a CAT convertor efficiency problem. You may have a flaky CAT and one that is acting up will cause misfires both random and fixed.

Just to be sure on the 02 sensors remove them both and clean with carb cleaner. Make sure you can see light through the vanes.

What year is this GTP and how many miles?

Thanks to all of you who responded. Sorry for the delay. I was rushing to get out of town for my grand farthers funeral. I put AC Delco plugs in before I left, and the car instantly ran wicked good. No more problems for about 200 miles or so. Then it started to come back, now I seem to be back where I started. I am not sure what this indicates. I am going to have the coil pack and plug wires for cylinder 4 checked and replaced if necessary.
My car is a 98' GTP with 96K miles. It had bosch plugs in it and removing them I thought had solved the problem. Is there any way to test the O2 sensors or the Cat without actually replacing them.
Thanks Jason

Are you saying that a misfore will not cause the P0420 code to be triggered? If not, how about Vise Versa, can the P0420 cause the misfire code to be thrown? How long do CATs last generally?

What bnaylor was saying is that the P0420 can be present without a misfire, and his definition of that code is correct. A bad misfire can cause that code, because the unburnt fuel entering the exhaust can overwhelm the converter's ability to process unburnt hydrocarbons, which can trip that code. Since the misfireproblem came back, if it's still indicating a misfire on cyl-4 (P0304), did you change the wires also? If the wires have the cylinder #'s on them, they are still OEM, and are past due for replacement even if they visually look good. If only the P0420 came back, you might have a bad convertor.

Rich's explanation is correct. It can go either way. Misfires can damage CATs and cause efficiency problems setting DTC P0420 and flaky CATs can cause misfires. Another scenario is the CAT causing misfires with no misfire DTC codes. Typically a bad CAT may generate P0300 random misfire but fixed pose a different problem and course of action. In your case, I would concentrate on the fixed misfire at cylinder 4 (P0304). Spark plug, wire, respective coil pack first. After that, fuel injector since that can cause a fixed misfire. If all else fails then look at a defective CAT.


How long do CATs last generally?

It varies. They can fail with under 50K miles or last a long time. Most of the time when they fail it will probably be out of the GM emissions warranty coverage so the owner is stuck with the repair bill. The CAT in my GTP failed at 70K miles. Uncorrected misfires damage CATs because the internal temperature exceeds 1200 degrees F.

Just FYI Bob, he typo'ed the code, it was a P0304, not P0301. He PM'ed me about the correction (see post#3) but forgot to mention it in his response (post #5) in this thread. Just wanted to mention it so we're all on the same page.

Just FYI Bob, he typo'ed the code, it was a P0304, not P0301. He PM'ed me about the correction (see post#3) but forgot to mention it in his response (post #5) in this thread. Just wanted to mention it so we're all on the same page.

Thanks for the heads up Rich. :thumbsup: