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2005 Pontiac Grand Prix GT code P0420

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deerslayer1985

New member
My car popped an engine code this evening on the way home from the store. I scanned it with my scanner when I got home code P0420. My scanner doesnt really tell what all the causes could be. There seems to be no miss, or power loss. Just a check engine light. Anybody know what this could be? Thanks for any help.
 


Pretty simple, the computer is saying the cat converter is not doing a good enough job at cleaning the exhaust gases flowing through it. It is now seeing more the allowed limit of oxygen/nitrogen etc etc and the rear o2 sensor is picking that up, thus the code for the cat.

If everything else is up to snuff with no other problems the converter is going to the cause. If it's far enough gone you can smell a rotton egg type smell from the tail pipe. Your options are to replace the car with a quality unit or space the rear o2 sensor out of the exhaust gases to try and stop the code from returning.
 
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What is considered a quality cat? Like name brand flowmaster, magnaflow, etc.....I see there are direct fit cats that bolt in, and universal cats the weld, or clamp in. What is the best way to go?
 
A flowmaster or magnflow cat are normally high flow units with performance in mind but not as good as a factory type cat converter. They have less cell counts and less overall material that won't work well to remedy the code problem. They work great but not for this application.

A new AC delco unit would of course be the best option but they cost way more money than some other brands. The brand that me and several others have had the best luck with in the Walker Cat converters. I've installed 2 so far on grand prix's and not one has tripped the light. The first one was on the car for over 2 years and this past one has been installed for nearly a year now with no issues at all.
 
temp fix to see if its just contaminated is go to Advance, Autozone,Oreilly, NAPA and as for them to get you a bottle of Mr. Gasket Cataclean. I like to make sure previous tank in car was premium fuel(seems to actually succeed more times than not to resurrect used cats) and follow bottle instructions but use whole bottle. THEN after treatment refill with premium again. Seems to help on cars that burnt oil and started contaminating the cats.
 
deerslayer1985 said:
What is considered a quality cat? Like name brand flowmaster, magnaflow, etc.....I see there are direct fit cats that bolt in, and universal cats the weld, or clamp in. What is the best way to go?
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Installed this 6 years ago and it is still doing its job.....

Catalytic_Converter_006.JPG


Catalytic_Converter_007.JPG
 


I had a buddy of mine tell me to just clear the code. He said it has happened to him.....Dont know if that is something that can really happen or not. I cleared the light that night. It hasnt came back on since. Do I drive it until it comes back on before buying a cat to make sure? It hasnt lost any power.....at least not enough that I can tell. It doesnt have a smell, and gas mileage seems to be good. Average around 25 mpg, higher on straight highway.
 
Cleaning a catalytic converter is a waste of money, time, and only supports businesses based on snake oil. Look up how catalytic converters work and you'll see why. Cats are common to go bad on these.

Last time I checked in the tech manual, takes around 150 miles of driving for the computer to run long fuel trims to set this code, so likely, it'll be back sooner rather than later.

I second Bronco on this brand, they're good. Good luck!
 
Ok thanks for the help guys. I have already put another 50 or 60 miles on it. I will go ahead and drive it this week and see if the light comes back on. If it does I will be getting a walker cat. Thanks for the help.
 
Magnaflow also makes a good hiflow cat, you need to order the Magnaflow HM high metal content cat. Before the switch to obd2 e testing in my area I had to have a sniff test done on my modded gtp and the the magnaflow HM was what I went with over going back to a factory cat. I believe the HM is good enough for California spec emissions as well.
 
deerslayer1985 said:
I had a buddy of mine tell me to just clear the code. He said it has happened to him.....Dont know if that is something that can really happen or not. I cleared the light that night. It hasnt came back on since. Do I drive it until it comes back on before buying a cat to make sure?
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For sure, if the light stays off, don't start replacing parts. Plan on it coming back on though.
 


wstefan20 said:
Cleaning a catalytic converter is a waste of money, time, and only supports businesses based on snake oil. Look up how catalytic converters work and you'll see why. Cats are common to go bad on these.

Last time I checked in the tech manual, takes around 150 miles of driving for the computer to run long fuel trims to set this code, so likely, it'll be back sooner rather than later.

I second Bronco on this brand, they're good. Good luck!
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The Cataclean isn't a "cleaner" per se. It's product that was formulated to cause converter to run at a higher temp so as to burn off any excess contamination. Like burnt oil, silicone contamination, or crap fuel additives. It's meant to help a contaminated converter come back to with in service limits. If the substrate is damaged or missing it won't help. I use rarely but it helps if an engine has oil consumption, ethylene glycol contamination. Etc. Many try to say use laquer thinner, bleach, etc but this was developed to help the converter process the exhaust. See the converter "stores" oxygen to maintain proper reaction after the unit "lights off" which means getting above 600-650 degrees.
Contamination tends to keep it from heating up and burning off any contamination. Again it's a cheap way to verify if converter is contaminated or just plain damaged. I have used it with great success and have actually proven converter damage vs converter failure.
I mean heck some of you will add Lord knows what to engines and trans to try to "save" them but you can't believe in a product actually developed to prove JUST what I stated.
 
Good input, and if your car is dumping oil on your cat, then this could be a band-aid, or a fix after you correct the real problem.

According to their website, cataclean is comprised of:
Acetone 10% to 30%
Xylene 30% to 60%
Isopropanol 10% to 30%
and Light Petroleum Distilates

All I'm saying is that this is a CLEANER, and if you need a cleaner for the catalytic converter, you also need to fix why it's dirty in the first place. I'm just not a fan of "band-aids".
 
Serious engine trouble, worn piston rings, etc. Basically, you'd know if it was happening because you'd have a host of other issues. The likelihood of you having a "dirty catalytic converter" and this cleaner doing anything permanent is slim to none. Don't waste your money on band aids.

As far as what causes them to go "bad" boils down to either poor manufacturing, engineering, the catalyst being depleted, or physical damage.

If it's a brand new cat causing this code, then it doesn't mean your catalytic converter isn't working, it could mean how it was designed is not close enough to spec and causing different readings then it expects. Only way to know this is viewing live data.

For instance, after installing speed daddy headers and a high flow catalytic converter, I got this code. Turns out it was working fine, but had the wrong af ratio and setting the code. Once I had my pcm tuned, poof no code. That's how to fix it without a band aid.

However, I've also bought cheap cats that were underperforming from day one and threw this code. Likely, it's the catalytic converter though
 
Check engine light still hasnt come back on, but my wife said it smelled like rotten eggs when she got out of the car today.......So I guess that is a sign. Someone told me putting a high flow cat on would give you better gas mileage. Is there any truth to that? I mentioned that it might pop the code if it doesnt flow the same as stock....He said something about a spacer for something. What would that be?
 


Hm... usually the smell of rotten eggs is burning coolant.

I'd still keep a look out for the code, but keep an eye on your coolant.

In my experience, high flow cats don't increase gas mileage, but most of them will set this code.
 
Never had that happen before, but yeah, I would agree with your conclusion being the catalytic converter.

Needless to say, no type or amount of "cleaner" will fix this. It smells like that because the catalyst is depleted and no longer converting the sulfides in your gas.

I would prepare to replace it...
 
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