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Discussion Starter · #1 ·
Okay so I have a 2005 Hyundai Elantra GT with California emissions, and ever since I've had it, it's had p0420 code. And it never really impacted performance, but recently it feels gutless and I know for a fact it's running lean because I pulled the plugs and they were white. I've replaced plugs and wires and the O2 sensor before the CAT(the one on the manifold) and I cleared the code and it hasn't came back on yet. But here's what I noticed when I pulled apart the exhaust: the first manifold CAT looks like it melted, and the second CAT is just completely gone. I'm sure the gutless feeling is from how lean its running. And I have a feeling it has something to do with the CATs and the O2 sensors. But I'm not real sure on how it all works together. Could somebody pleeeeaaaasssee tell me why my car is running lean? And what I need to do to fix it?
 

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California...

OK, this is how it works.
First O2 measures what exhaust gases you get and it controls what you get injected to the combustion chamber. In other words - this is the main O2 sensor. It is before CAT.
After the CAT you get another one that measures how the CAT works. It will not affect the way the engine works, UNLESS Cal emission makes the car to work slightly different than I know...

Melted - means too much heat. That would rather suggest too much fuel coming out and burning on the cat.
 

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Discussion Starter · #3 ·
See that's what I had thought, that the first 02 sensor was bad so I replaced it, and I still have to same problem. So my question is, what's making it run lean? I sprayed carb clean around the injectors to see if the engine would get excited, to see if the injectors were sucking air past them
 

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0420 is a cat efficiency code.. normal program logic/fuel trim will leave plugs fairly clean..

Sluggish.. you state the cat looks melted.. then drop the front pipe off bottom of the primary cat and look up in at the brick.. should be nice flat looking-well defined honeycomb.. any melting/displacement indicates brick been overheat and melt, hence the 0420 cat efficiency code.

Any missing material from the primary cat will flow down stream and clog the down stream cat there at rear of front pipe.. remove front pipe and shake the debris out to unclog the down stream cat so the engine/exhaust can breath again.
 

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Discussion Starter · #6 ·
Well I already thought of that. I did already look up through the first cat by dropping the pipe. And it is melted, there's only a little left. And the second cat was gone. I looked down the pipe and its hollow. And so I took the muffler off and of course its full of cat debris. So I just took it out completely and it still runs sluggish and lean. Maybe with out the first cat, the O2 sensors are making it run lean??? I read somewhere that if the O2 sensor goes bad it can actually destroy that first Cat. As to if that's true or not, I don't have a clue
 

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Misfires kill a cat..

Replace the exhaust manifold/primary cat, make sure exhaust is not restricted.
 

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Well I already thought of that. I did already look up through the first cat by dropping the pipe. And it is melted, there's only a little left. And the second cat was gone. I looked down the pipe and its hollow. And so I took the muffler off and of course its full of cat debris. So I just took it out completely and it still runs sluggish and lean. Maybe with out the first cat, the O2 sensors are making it run lean??? I read somewhere that if the O2 sensor goes bad it can actually destroy that first Cat. As to if that's true or not, I don't have a clue
It's not gonna fix itself, it has default running codes stored in ECM
that it's working from.

IMO, You need to idle it in Park up to operating temp. into "closed loop O2 sensor learning mode"

(Out of computer controlled cold engine "open loop" mode) Then disconnect the Neg battery cable for 5 mins, or so, then put the cable back on, start it up, and drive it like you stole it. You're probably running in a very lean mode, with open exhaust, and now the O2 sensor will richen up the mixture to suit! Once you get a new cat, & or muffler installed, you'll probably have to do this again.

Here, read this:http://www.thirdgen.org/open-loop-closed-loop
 
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