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Discussion Starter · #1 ·
Hi guys. I recently got a diagnostic computer check on my 2005 Hyundai Elantra 4DSD that has over 95,000 miles and got the code P0420 showing the following:

1. Catalytic converter defective due to
2. Engine misfire or running condition
3. Large vacuum leak
4. Engine oil leakage into exhaust valve guide seals, piston rings

The car is running fine. No excessive gas consumption, no difficulty starting up or stalling, but the check engine light has turned on every 3-4 weeks and stays on for a few days and then turns off again. Two mechanics have diagnosed the car engine and claim I need a new front catalytic converter.

There is a mechanic on YouTube that claims he has corrected the problem by pouring a gallon of lacquer thinner into a half full gas tank and running the car for 150 miles at highway speeds. Why? He claims code P0420 may show up when the catalytic converter is only 5% inefficient and that the lacquer thinner will sometimes clean the catalytic converter so that it can be efficient once again. What do you think? try the cleaner? Replace the oxygen sensors? Or simply replace the front catalytic converter?

Bill
 

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LOL I have seen that guy, he's funny, some of his stuff is good knowledge.

It probably needs the converter, although he probably is correct about only 5% is bad. Rockauto.com usually has good prices and the old one you can sell easily to a "scrapper" for maybe $100.

Or you can try his lacquer trick, but maybe do it through a vacuum line instead of the gas tank, seeing as lacquer is used in most fuel induction chemicals using this process. Good luck.
 

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Is your gas tank one of the newer plastic kind? Just asking. Some are. Not sure what the properties of lacquer thinner might do in this case. I'd just pony up for the converter. Snake oil remedies scare the **** outta me. Not that they're all designed to lighten your wallet, mind you. I just don't care to take the gamble.
 

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Plug in GDS and watch Ox activity in current data -> graph mode.

If front and rear ox pattern look a lot alike, then cat is dead



If front move back-forth and rear stays pretty well flat line with only real activity as throttle moved, then cat fine..


NOTE - These is not actual GDS screenshot, but they do represent what we looking for as far as verifying catalyst being operative via ox sensor activity
 

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Discussion Starter · #5 ·
Thank you all for your input. My car is getting close to the 100,000 mile mark and I plan to keep my car for some time, because I cannot afford to buy a new one. This Elantra has been one of the most dependable cars I have owned. I am the original owner and have been really good with the maintenance. Gas mileage is great and the car runs well. I was trying to see if I could deal with the catalytic converter issue without spending a lot of money, but I now think that it is better to invest in the car for the long haul.

I am not going to spend a lot of time and money trying to figure out if the the catalytic converter is still good or not. If I keep the car long enough (and I plan to hold on to it for a few more years), eventually parts of the exhaust system will degrade and may have to be replaced. Right now my car is showing signs of some exhaust system problems and I'd rather deal with the whole exhaust system now, rather than wait until it gets worse and change things as needed. I just think its easier to replace the whole exhaust system and not deal with the downtime and cost of later repairs. I can't complain. The exhaust system has served me well for close to 100,000 miles. Its time for a fresh start.

I am going to replace the whole exhaust system and I will do the job myself. If anyone has any videos of the job please post the link.

By the way, is it possible to remove the cat at the exhaust manifold and drop it to the ground with the whole exhaust system? That way I would not have to bother with the possibly fused bolts and flanges along the exhaust system which would make for a much quicker job. What do you think?

Bill
 

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Ifyou've done exhaust replacement before then you know what you may be getting into. But if you haven't, they can be a PITA...with rusted screws and bolts, aftermarket parts not always fitting exact, and clamping (prone to leaks more) vs. welding are all things to keep in mind. Yes going with whole system may be the way to go but jacking car high enough is also an issue when DIYing and unbolting from manifold can also be just as frustrating.
 

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Exhaust manifold and primary catalyst is all 1 part.. then front pipe bolt onto bottom of exhaust manifold and carry on back to next flange for rear pipe to bolt together, then center pipe carry on to rear where next flange is, with muffler bolted on and tailpipe stick out below bumper.Pics is rough view of the parts, there be gasket between manifold to head, 3 gasket for pipes, 6 nut, maybe ox sensors if seized, 2005 use 6 wire at manifold, it pricey..



 

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It looks like the cat died. I would go to aftermarket headers, and add an aftermarket cat. That way of the cat dies again, it is easier to replace. And cheaper.

Several of the junk yards,,,sorry, recycled auto part yards in the area, pull the cats and sell them, telling you they are required by law. That's BS! I have never found any such Federal or state law in the South about this.
 

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billx21, just FYI, I had the P0420 for quite a while. Tried everything but the cat. Finally gave up and replaced it and that fixed the problem finally. Duh. But honestly, it wasn't very hard and the parts weren't horribly expensive either, got them online. I don't think I even had to use an impact wrench on the manifold bolts. I only replaced the front half though. From the resonator on back was still in good shape. I did need an impact to rip those rusty bolts off to separate the pipe. If you're replacing the whole thing front to back though, it shouldn't be very hard at all. Like they said, mostly a matter of getting the car up in the air far enough that your nose isn't scraping the undercarriage and a bunch of rust in your eyes.
 

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Sounds like the problem we were getting earlier this year on my daughter's '06 GLS. I had the flex pipe replaced and it seems to have taken care of it. FWIT the dealer did this for me - very reasonably - must be the three Hyundai's I have.
 
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