P0420 code for check engine light...HELP!!!!
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Thread: P0420 code for check engine light...HELP!!!!

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    Default P0420 code for check engine light...HELP!!!!

    Ok, I have a 2004 Hyundai Elantra that I bought used 8 months ago. It has just over 72,000 miles on it and the check engine light came on. I took it to Checker's and they gave me the P0420 Catalyst System Efficiency Below Threshold (Bank 1) code. I asked for the english version and he said O2 sensor or Cat and reset the code. It came back on tonight while I was at work (delivering pizzas) and one of the other drivers suggested to tighten my gas cap. It turned it off!! So was that the problem the whole time or was I just lucky this time? Also, if it is the O2 sensor or cat will my continued driving totally destroy my car? And is there any way to find out which it is or do I just have to suck it up and pay Hyundai $100 for a diagnosis?

    Any help would be really appreciated because I am really afraid that if I continue to work that my car is going to blow up or something but if I don't work I won't have the money to fix it. HELP!!!!!

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  3. #2
    turbo envy only1db has picked up a wrench only1db's Avatar
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    Default Re: P0420 code for check engine light...HELP!!!!

    probably the front 02 sensor....or even the rear one

    you wont know till its graphed and diagnosed as such.


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    Super Moderator hyunelan2 knows a lot about Elantras hyunelan2's Avatar
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    Default Re: P0420 code for check engine light...HELP!!!!

    If tightening the gas cap fixed it, I wouldn't do anything unless the light comes back on. That's gotta be the most common CEL people ask about. Just make sure you turn the gas cap "3 clicks" whenever you put it on.


    2010 Equator Sport | 2002 Elantra GLS (sold) | 2003 Tiburon GT | 2004 Santa Fe GLS


  5. #4

    Default Re: P0420 code for check engine light...HELP!!!!

    Quote Originally Posted by only1db View Post
    probably the front 02 sensor....or even the rear one

    Nah, the front o2 doesn't monitor the cat, only the rear o2 does. So if anything, it's the cat that's no good. If the rear o2 was bad, the code would most likely be bank 1, sensor 2 low volts or slow responce or something like that. When you get a cat efficiency code, it's most likely a bad cat. But if the CEL isn't on or went off by itself, then there is no problem. So don't worry unless it comes back on.
    Last edited by DJ Hellfire; 01-13-2008 at 01:26 PM.
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    Senior Member 2001VE has not yet proved knowledgeable 2001VE's Avatar
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    Default Re: P0420 code for check engine light...HELP!!!!

    Not a bad idea to calculate your gas milage. Sometimes a problem showing a code well drag it down so much it will make financial sense to fix it right away. Especially true if you are driving for a living. I have had worn out cats on vehicles ; it usually just made me fail an emmission test but didn't drastically change gas milage. MY 2$.

  7. #6

    Default Re: P0420 code for check engine light...HELP!!!!

    Thank you to everyone who replied, it has definitely taken a load off my shoulders. I litterally put gas in my car everyday so I will make sure that the gas cap is on tight every time. If the light comes on again I will take it in.

    Another random question if I could, is it better to run your gas tank out and fill from empty or fill up the top? Right now I'm only putting in 1/4 tank before work and it's empty before the night is over. At this rate I'll be needing to ask how to fix the low gas light bulb. ^_^

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    Senior Member 2001VE has not yet proved knowledgeable 2001VE's Avatar
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    Default Re: P0420 code for check engine light...HELP!!!!

    Probably lots of different opinions on this. In northern areas its probably better to keep it full during winter to reduce condensation ; especially it car sits quite a bit. But gas is fairly heavy and hauling around a full tank all the time could slightly affect milage (crazy small like 0.1 MPG , but over a year could still add up). The gas light bulb is probably an LED with a life span greater than the car; not an issue. Personally I like to price watch like hell and fill up when I think they are about to jump up. Over $4 Cdn. for a US gallon right now; ouch. Paid only $3.29 US in Grand Island N.Y. two weeks ago.

  9. #8

    Default Re: P0420 code for check engine light...HELP!!!!

    Quote Originally Posted by 2001VE View Post
    I have had worn out cats on vehicles ; it usually just made me fail an emmission test but didn't drastically change gas milage. MY 2$.
    All it does is reduce emissions. A bad car won't affect gas mileage unless it's clogged.
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    turbo envy only1db has picked up a wrench only1db's Avatar
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    Default Re: P0420 code for check engine light...HELP!!!!

    i have yet to see a failed cat on an XD....i have seen one come through our dealership and that was a santa fe with almost 200k on it...that i had the pleasure of replacing (bank one)

    what i have seen are multiple 02 failures....i would go to the dealership and let them do the final diag on this one. the car needs to be warmed up to operating temp and then the 02's graphed.


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    Exclamation Re: P0420 code for check engine light...HELP!!!!

    Quote Originally Posted by DJ Hellfire View Post
    the front o2 doesn't monitor the cat, only the rear o2 does.
    Actually, I believe the computer monitors the difference in oxygen between the first and second oxygen sensors and uses that to see if the catalytic converter is functioning properly.

    That way, you could get a code for the first sensor (bad sensor or heater), second sensor (bad sensor or heater), and / or catalytic converter efficiency (second O2 sensor reading the same as the first O2 sensor).
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    Mike and Diana reallyflycouple has not yet proved knowledgeable reallyflycouple's Avatar
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    Default Re: P0420 code for check engine light...HELP!!!!

    I agree, Hyundai has always had issues with experimental O2 sensors. I remember always changing V6 O2's and flashing the ECM's. I think it was the same for me, I had to replace cats on Tib's and Sonata 2.7's. Not the 4 cylinders. Not to say that there isn't a problem. Scan it, watch the graph of the primary and secondary O2. The primary should range from 0v to 1.0v roughly 2 times a second (depending on it's condition). The secondary should almost stay steady unless you spike your throttle. It's so easy to diagnose these issues! As long as you have a good scanner.

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    turbo envy only1db has picked up a wrench only1db's Avatar
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    Default Re: P0420 code for check engine light...HELP!!!!

    yeah like the hyundai gds...hahah


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    Default Re: P0420 code for check engine light...HELP!!!!

    Quote Originally Posted by only1db View Post
    yeah like the hyundai gds...hahah
    And those are pricey. I'm not going to mention how much mine was.

    "Childhood is measured by sounds and smells and sights before the dark hour of reason grows"John Betjeman

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    Default Re: P0420 code for check engine light...HELP!!!!

    I got an estimate today for parts and labor to replace the O2 sensors and I wanted to run it past you guys. This is the first car I've ever really done maintenence on and I don't know a thing about cars. I was told that there are two O2 sensors, one upstream and one downstream, and just for the parts one is $119 and the other $125 and then throw in like $90 for labor. Based on the code I'm getting can you tell if its one or the other or would it just be better to replace them both at once? I was also told that I need to replace my 3rd AC belt and air filter, can I do that at home?

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    turbo envy only1db has picked up a wrench only1db's Avatar
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    Default Re: P0420 code for check engine light...HELP!!!!

    you will be able to graph the 02 sensors to see which one is faulty...you should not just throw parts at it....thats being a grease monkey not a tech.


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    Large Member kylemorg has not yet proved knowledgeable kylemorg's Avatar
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    Thumbs down Re: P0420 code for check engine light...HELP!!!!

    Quote Originally Posted by krodammi View Post
    Based on the code I'm getting can you tell if its one or the other...
    Did you not read the earlier posts in this thread that said that the only way you'll know what part is bad for sure is to take it in to Hyundai? Please re-read the posts in the thread you started before asking the same question multiple times.
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    Full Member dmiller66 has not yet proved knowledgeable
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    Default Re: P0420 code for check engine light...HELP!!!!

    Hey,
    My experience has been that if you have a bad sensor, it will signal its own code. My car just gave me a P0420 about a month ago, after I replaced the 02 sensors, but didnt replace the cat converter. Once I replaced the converter/ exhaust manifold ($165) the light never came back on. The converters are cheap if you shop around. I got mine at: http://www.trademotion.com/partlocat...02&catalogid=1
    hope this helps.

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    Ultimate Post Whore cbogg knows a lot about Elantras cbogg's Avatar
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    Default Re: P0420 code for check engine light...HELP!!!!

    Quote Originally Posted by 2001VE View Post
    Probably lots of different opinions on this. In northern areas its probably better to keep it full during winter to reduce condensation ; especially it car sits quite a bit. But gas is fairly heavy and hauling around a full tank all the time could slightly affect milage (crazy small like 0.1 MPG , but over a year could still add up). The gas light bulb is probably an LED with a life span greater than the car; not an issue. Personally I like to price watch like hell and fill up when I think they are about to jump up. Over $4 Cdn. for a US gallon right now; ouch. Paid only $3.29 US in Grand Island N.Y. two weeks ago.
    Dude you were in GI? Wish I had known, that's only like 1/2 hour from here! And being that the wife is "Canuckian" it's kinda nice to see y'all around once in awhile. It could have been a mini-meet (just two cars, cuz I'm not sure of anyone else from where I am that has an elantra, and cares about it).

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    Newbie tannatt has not yet proved knowledgeable
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    Default Re: P0420 code for check engine light...HELP!!!!

    My wife's 2002 Hyundai Elantra has the Check Engine Light illuminated. I scanned the OBDII for codes and received a P0139 / O2 Sensor Slow Response, Bank 1 Sensor 2. I understand the Bank 1 (if there was more than one bank on an in-line 4, I'd be concerned), and I understand the Sensor 2 (downstream, great).

    According to conventional knowledge, the upstream sensor is before the cat (usually in the exhaust manifold), and the downstream sensor is after the cat - fine.

    My problem is that I have one O2 sensor in the exhaust manifold and a second between the flex pipe and the cat - that's it. So am I looking at a car with two upstream sensors and no downstream sensors? I am close to pulling the car off of its jack trying to get the second sensor (between the flex pipe and the cat) to loosen up, so I'd like to know that I have the correct part in hand before crushing myself.

    My local Hyundai dealership has graced me with the answer, "2002 Elantra? I think that has 4 O2 sensors..." I believe that translates to "give us your money and we'll fix it"

    Any help would be greatly appreciated!

    -Sam

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