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Unread 05-30-2012, 06:00 PM   #1
platinum5.9
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The end all p0420 code thread...

I have been dealing with the dreaded p0420 code in my 03 Grand for some time now. Its a 4.7 ho with the two mini precats, full size Magnaflow cat and 4 O2 sensors. I have read numerous articles that say its the drivers side top O2 sensor, some that say the pre-cats need replaced, and some that say it could be caused by a exhaust leak causing the O2 to read incorrect. I need input from people that have had the issues and managed to correct them and people who maybe do this for a living and have more knowledge than me and the mechanics in this small town. NYS inspection it has arrived and I need to get this thing to pass inspection. Any help would be great and Thank you in advance.

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Unread 05-30-2012, 06:39 PM   #2
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That code means that the switching frequencies of the upstream and downstream O2 sensors are essentially the same. The same switching frequency means the exhaust gas passing each sensor has the same composition and that the cat is not doing its job. A bad O2 sensor won't have a switching frequency that matches the frequency of a good O2 sensor. That codes can't be blamed on a faulty O2 sensor. You are due for a pre-cat replacement.
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Unread 05-30-2012, 06:55 PM   #3
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That was my original thought but after being clouded with a the other info out there I am so confused....that's why I posted this thread. So the drivers side pre-cat is bad and needs to be replaced? Can I use a replacement mini cat from magnaflow?
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Unread 05-30-2012, 07:55 PM   #4
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Quote:
Originally Posted by platinum5.9 View Post
That was my original thought but after being clouded with a the other info out there I am so confused....that's why I posted this thread. So the drivers side pre-cat is bad and needs to be replaced? Can I use a replacement mini cat from magnaflow?
I would replace them both for equal backpressure.
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Unread 05-30-2012, 09:40 PM   #5
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I would contact Kolak about which Magnaflow minicats to order. It would be a good idea to go ahead and replace both sides.
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Unread 05-31-2012, 10:28 AM   #6
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Yeah I've talked to him already and if i decide to go that route i'll replace both. I read somewhere to run some seaform thru the gas and maybe that would help. No luck with that method.
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Unread 05-31-2012, 08:05 PM   #7
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Originally Posted by platinum5.9 View Post
Yeah I've talked to him already and if i decide to go that route i'll replace both. I read somewhere to run some seaform thru the gas and maybe that would help. No luck with that method.
gotta love ny inspection! i had the same thing on two gm cars in the family, took almost a dozen tries to catch that "window" to get both to pass, tried nearly everything, both had new exhausts front to back less than a yr old, turns out both had leaky intake gaskets, let tiny amounts of coolant into the intake, so it was basically fouling up the cats, found a trick online that works to clean the cats, as they rarely clog completely or blow out, just get very dirty or carbon'd up, with a half tank, throw in a gallon of lacquer thinner, and then drive at highway speeds for 150 miles or so, its essentially like throwing in high octane/fuel system cleaners, but much more effective, and the funny thing about the 420 code is that it can trip when the cats fall below 95% efficiency, so they usually arent bad just partially clogged, if they were shot, then u would know it! no power and constant stalling, combined with higher than usual engine temps, just a little food for thought, hopefully it helps u some
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Unread 06-01-2012, 11:03 AM   #8
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I sent a PM to oldgeezer over at jeepsunlimited.com because he is very knowledgable. This is what he sent back and seems to be the answer I am looking for. "Thank you for your kind words. That P0420 codes is one of the most unambiguous of all the diagnostic trouble codes. That code is thrown when the upstream and downstream O2 sensors both have the same "switching frequency." That means that the cat is doing very little to change the exhaust gas composition. In other words, the cat needs to be replaced. FWIW, if one or the other O2 sensor is bad, the switching frequency can't be the same. That's why the code is unambiguous; it can't be generated by a bad O2 sensor as some people would hope it could"
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Unread 06-01-2012, 11:39 AM   #9
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Quote:
Originally Posted by berndog67

gotta love ny inspection! i had the same thing on two gm cars in the family, took almost a dozen tries to catch that "window" to get both to pass, tried nearly everything, both had new exhausts front to back less than a yr old, turns out both had leaky intake gaskets, let tiny amounts of coolant into the intake, so it was basically fouling up the cats, found a trick online that works to clean the cats, as they rarely clog completely or blow out, just get very dirty or carbon'd up, with a half tank, throw in a gallon of lacquer thinner, and then drive at highway speeds for 150 miles or so, its essentially like throwing in high octane/fuel system cleaners, but much more effective, and the funny thing about the 420 code is that it can trip when the cats fall below 95% efficiency, so they usually arent bad just partially clogged, if they were shot, then u would know it! no power and constant stalling, combined with higher than usual engine temps, just a little food for thought, hopefully it helps u some
I have also heard that this will clean out a cat. Saw a reputable repair guy who does YouTube simple fix videos talk about this. Simple to try with no risk really.
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Unread 06-01-2012, 01:37 PM   #10
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Think about it a minute, stuff like lacquer thinner or fuel system system cleaners go into the gas tank and can do some cleaning until it is sprayed into the combustion chamber. Then it is burned just like the gasoline and becomes exhaust gas just like the gasoline. Once it is burned, it can't clean anything in the exhaust system. It certainly can't clean a catalytic converter. Sorry guys, this idea just doesn't make sense.
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Unread 06-01-2012, 06:48 PM   #11
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Think about it a minute, stuff like lacquer thinner or fuel system system cleaners go into the gas tank and can do some cleaning until it is sprayed into the combustion chamber. Then it is burned just like the gasoline and becomes exhaust gas just like the gasoline. Once it is burned, it can't clean anything in the exhaust system. It certainly can't clean a catalytic converter. Sorry guys, this idea just doesn't make sense.
they do work, the lacquer thinner trick worked on both vehicles i have tried it on so far, to keep it basic, it essentially is like running very clean, higher octane fuel, take apart any daily driver, u would be surprised how dirty your entire fuel system, engine and exhaust can get from low octane and winter fuels, for a real world example, my uncle has a 69 prostreet camaro, uses nothing but 110 race fuel and has put on nearly 20,000 miles since that motor was put in it about 8 yrs ago, he took it out and went to rebuild it and build it up for an upcoming blower and found absolutely no carbon or sludge buildup anywhere, it looked as new as the day it was originally built, so dont listen to the people who say all those cleaners and fuel additives are just a waste of money, because if u throw one in every now and then, especially during the winter months and use higher octane fuels and u will def have less emission related problems, less sludge and carbon build up, better fuel economy, will last longer, and less down time and outta pocket repairs!
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Unread 06-01-2012, 09:12 PM   #12
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I saw this recomended by Scotty Kilmer.. 30 year mechanic does quick videos on YouTube. You might not like his style on video... a little like the Oxy Clean guy. But he has some good insights into car repair. Here was the video about using the Thinner to fix a cat problem.

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Unread 06-01-2012, 09:18 PM   #13
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I'll try again. Forget about intake and combustion chamber and think only about the exhaust system since somehow the catalytic converter is supposed to be cleaned. However, a clogged cat causes one kind of problem, it restricts exhaust flow and kills performance. The other problem with cats is the catalyst can be poisoned which means it does not clean up the exhaust gases. It's this condition that corresponds to low converter efficiency. Any organic solvent and cleaners that are mixed with the fuel will get burned just like the gasoline. That burned up lacquer thinner will not do any more for the cat than burned up gasoline, regardless of the gasoline octane.

It doesn't make sense that a cat can be cleaned or that the catalyst can be rejuvenated that way, and as Judge Judy would say, "if it doesn't make sense, it isn't true."
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Unread 06-01-2012, 09:26 PM   #14
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ive never heard of cleaning it until now, hmm, ill have to try that on my bmw, its been throwing that code for about 3 years now
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Unread 06-01-2012, 10:43 PM   #15
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Originally Posted by rm2001wj View Post
I'll try again. Forget about intake and combustion chamber and think only about the exhaust system since somehow the catalytic converter is supposed to be cleaned. However, a clogged cat causes one kind of problem, it restricts exhaust flow and kills performance. The other problem with cats is the catalyst can be poisoned which means it does not clean up the exhaust gases. It's this condition that corresponds to low converter efficiency. Any organic solvent and cleaners that are mixed with the fuel will get burned just like the gasoline. That burned up lacquer thinner will not do any more for the cat than burned up gasoline, regardless of the gasoline octane.

It doesn't make sense that a cat can be cleaned or that the catalyst can be rejuvenated that way, and as Judge Judy would say, "if it doesn't make sense, it isn't true."
Except that it is NOT gasoline or even similar to it. And the burned products may be different. The gasses produced may have a cleaning effect. I can certainly tell you that High Octane gasoline smells NOTHING like regular gas. I guess you would have to try it to find out. But what is the risk. You can spend BIG $$$ for a new cat or spend a small amount on the possible fix... not much to lose. And I have seen several reports of people saying it works. Remember as he said in the video, a 5% loss in the cat can throw a code. A small amount of cleaning may fix that.
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