Thread: OBD codes P0420/P0300- little/no power past 4th gear, fuel in oil, sulfur in exhaust

2002 GT

Shadetree mechanic and I'm really bummed. This only started within the last couple weeks when I went to pass a semi on the freeway and the car started jerking after giving it a lot of gas very quickly.

My first guess on this is fuel delivery issues obviously. I pressure tested the fuel lines and it shoots up to 55 psi then loses pressure quickly. Fuel injectors, fuel pump/filter, fuel pressure regulator...however, today the car started idling VERY rough and I could smell rotten eggs coming from the exhaust. Am I correct in assuming that catalytic converter problems are not the problem per se, but instead a symptom of something else happening "upstream" in the exhaust system?

OBD-II code P0420 indicates a CAT problem, but P0300 indicates I have random cylinders misfiring. So am I also correct to assume that fuel delivery issues are causing the misfires and thus the CAT problems or am I way off?

I smell a hint of gasoline on my dipstick and it takes a couple seconds of cranking to start, then dies quickly. On the second try it will stay running. Granted it's idling rough. It takes off from a stop fine, but when giving it gas in anything over like 3rd or 4th gear it just hesitates and jerks terribly. Wtf is happening here? Is it losing power because of fuel issues or misfires? Or is it both? Did I waste $100 on a fuel pump/filter combo?

I'll be replacing the fuel pump and filter when they arrive later this week (though somehow I'm skeptical that's even my issue here). Until then, it's my daily commuter (40 miles both ways, 5 days a week) because I can't get parts again until I get paid on the 1st. If I lose my car, I lose my job.

I don't have access to any professional diagnostic equipment. Aside from just adding oil and coolant, I have never really worked on any car, ever, as I have never had anyone to teach me. Recently, I swapped the upper intake manifold and lower intake manifold gaskets with aluminum Felpros by myself. Swapped water pump, oil/filter and coolant as well during that project. Watched Youtube videos and read a couple forum tutorials. I didn't find it terribly difficult, just time consuming, and this gave me a lot of confidence and I really want to learn everything about how this car works so I can just care for it myself. It just past 130K and I hope to keep it past 300K.

This is my second sixth-generation Grand Prix GT and I'm really in love with the cars and especially the engines. I think they're wonderfully engineered other than a few minor flaws that aren't that difficult to iron out yourself with a little willpower. This is also my first post here and I hope to be a member for the forseeable future! Down the road, I eventually plan on buying a 7th-gen GXP


I really do appreciate any guidance you guys could offer a new shadetree mechanic.

EDIT: I am aware my CAT is done for, I assumed that they go bad because unburned fuel causes the honeycomb filter in them to melt, break into pieces and thus plug them up, or is that wrong? What's the saying, "cat. converters don't die, they're murdered"? Is the fuel in my oil unrelated?

Sounds like your catalytic converter is plugged. Sulfur smell is a dead giveaway. A plugged catalytic converter can cause the car to hesitate when trying to accelerate and also can cause a p0300 code. Does your CEL flash when you try to accelerate? Remove your front o2 sensor from the manifold and take it for a spin, if it accelerates just fine you need a new cat.

likely a clogged cat, it can cause misfires and the cel to flash when you floor it.

Originally Posted by Scottydoggs

likely a clogged cat, it can cause misfires and the cel to flash when you floor it.

Ha ninjad

it happens time to time.

I'm aware the cat is bad, I'm wondering if the fuel in my oil is the cause and what the cause of that might be. The sulfur smell is only a recent development. I was led to believe unburned fuel is what makes the cats go bad in the first place.

my oil has always stunk of gas by 3000 miles. i call it normal as it happens with the new engine i got now

with manifold air pressure being higher due to an exhaust restriction the pcm will add extra fuel as it thinks your under higher load, cylinder washdown and extra fuel in the oil is an effect, not the cause

Originally Posted by timeshhift

I'm aware the cat is bad, I'm wondering if the fuel in my oil is the cause and what the cause of that might be. The sulfur smell is only a recent development. I was led to believe unburned fuel is what makes the cats go bad in the first place.

Originally Posted by Turbocharged400sbc

with manifold air pressure being higher due to an exhaust restriction the pcm will add extra fuel as it thinks your under higher load, cylinder washdown and extra fuel in the oil is an effect, not the cause

This^

Turned out to be combination of the fuel pressure regulator, fuel filter and ignition control module.

This started before I replaced the FPR. Then I replaced it. Seemed like a good idea since it was leaking fuel (Stealership said they "fixed" it) Anyways, this helped in lower gears. This also took away my P0420 AND P0300 codes as well which confused me. After this, the the problem slowly resurfaced, but the codes never came back.

Then I replaced the fuel filter. I could barely blow a little bit of air through it, even when blowing pretty hard. This helped immensely, as I noted on the way home, though she was still missing in higher gears.

The next morning I go to leave for work and back out of the driveway, then just barely get on the gas and she takes a huge **** in the middle of the road. I could start it after some 3-4 second cranking, but as soon as I gave it gas it stalled again. Managed to get it backed up against the curb.

So then I decided to swap out the now blackened soot-covered platinum plugs, (they were white the day before) for iridium XPs. Craaanked and started it, drove it around the block to make sure it ran, then parked it.

The NEXT DAY, I go to start it and it now doesn't turn over at all. I remember literally saying "what the ****" out loud. I've never heard of someone adding OEM parts to a car and have it make the problem worse, twice.

So I hooked up an in-line spark tester to the wires and I was getting a really weak light, as if there was barely any power going through it. I decided to have the ignition control module tested since it was free. Both stores tested it 5 times, failed all 10 tests.

Bought a new one, threw it on there, and the thing started better than the day I drove it off the lot. No misfires under load. Renee runs like a ****ing CHAMPION!

Originally Posted by Omega

Hey new to the forum. Figured I give it a go. I believe im in the right section. The thing that brought me to this sigh was the back review of spyder projectors for 04 grand prix. I saw someone was selling retro fix with alot better results. Second is Coilovers the was to go for a better ride? Also switch back LEDs i fixed hyper flashing in taillights after led replacement by add resistor and soddering it to the wires. Will that work for SWitch back leds for Front turn signals?

how about you start your own thread instead of jacking this guys thread with your none related issue?