Thread: Pontiac GP 08 - P0300 & P0420

I have a 2008 Base GP, 3800 engine, about 110,000 miles, daughters car.

A couple of weeks ago she got CEL pulled the code P0420 Catalytic converter, cleared the code, car seams to be running fine, I don't drive it much.

Today she got a flashing CEL and complained about no power and a lot of vibration. Pulled the code, P0300.

I don't have my live data scanner, trying to get a hold of my buddy to get it back. What are the steps to get this diagnosed?

Thanks,
John

Forgot to mention, after clearing the P0420, it came back after a few days, prior to getting the P0300.

Just started it, about 30 degrees outside, initially idled around 1500, then calmed down to about 900. Listened for for vacuum leak didn't hear anything. Tried revving the engine and it goes to 3500 RPM in Park and won't go any further.

Is that due to the cat clogged?

Thanks for your help.

Figured it was the cats as well, took it to a place called Iowa Muffler and Brake in Des Moines, they have always been straight with me (or so I think). He said the cats could be bad, but recommended fixing the misfire first, they recommended a small shop in the area, initially they thought it was a coil and stuck in a coil they knew was good, no go. They then determined it was the ICM that was bad, paid for the diag and headed to NAPA and picked up a new ICM, took a few minutes to install, fired it up and it instantly stalled, tried to start again it cranked longer than usual then fired up. Took it for a test drive and she drove great, neither P0300 or P0420 reappeared, I wouldn't be surprised if the P0420 reappeared.

Question for you guys, how often do the ICMs fail? The original failed and Pontiac replaced $1000, then it failed again, local shop replaced it $600, now it failed again. This one cost me $220, the top of the line one that NAPA had Delphi.

Thanks,
John

Plugs were not original, maybe 30k on them.

Yes ICM’s go bad and cause lots of weird things.

I was thinking coil or ICM. 110 k is pretty low miles for a cat to go. Most all of mine have went 200-300K.


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ICM's can also fail due to a cracked coil. Had that happen on my 00 GT. JY ICM and coils and it fired right up and has been running fine ever since. Have about 200K on those parts now.

Amazon sells a new AC delco ICM for $159


ACDelco D1977A GM Original Equipment Ignition Coil Interface Module https://www.amazon.com/dp/B0013G0IDY..._cdIpEb9349G0P




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Spent the weekend working on Jeeps with little to no internet access. Needed to have it going before I went to the guys weekend and didn't have time to wait for Amazon or Rock Auto to deliver. Good idea, about picking up a spare and a couple coils from the bone yard, may do that on my next trip.

The misfire is still gone, but, the P0420 has returned, Probably need to think about replacing the cat. Not in a big hurry, could be the car has been missing for a while, just not as bad, may attribute to a premature cat failure.

I see Rock Auto has several bolt in cats for around $200, what is the chance of those actually unbolting with out breaking the bolts off? A lot of rust in Iowa.

I bought this car in 2008, same day GM announced Pontiac was shutting down, overall been a pretty good car. My kids have driven it for the last 6 or 7 years , a lot of short trips to and from school. That may attribute to the cats going bad so soon.

Thanks for everyone's input, always appreciated!

John