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Discussion Starter · #1 ·
Hello,

Looking for some advice on the above codes. My Elantra's check engine light came on while driving on the freeway. The codes state (from Auto Zone's diagnosis) that this could be several issues. I currently am scheduled for a transmission service and engine tune up. Anyone have experience with these codes and the common reason they come up? I will say that I cannot detect a misfire or smoke of any color coming from the exhaust...


Thanks!
 

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Your catalytic converter has gone bad, or second o2 sensor.

You have a random misfire. Clear the codes. The random misfire may have just been a one time thing.

Google o2 spacer or anti fouler and try that. If it relieves the code it means your cat is no good. You can leave the spacer, but on some cars the car going bad means bad news for the internals of your engine.
 

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Discussion Starter · #3 ·
Thanks for your input. Now, when you speak of the "second" oxygen sensor, do you mean the upstream sensor (near the engine) or the downstream sensor (near the cat)? I don't think my cat is gone, the smell you get when that happens is not present, nor any odd colored smoke.. clear as day.
 

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How's your exhaust system? Leaks in the flex pipe can create conditions to set this code.

It basically means there's too much O2 in the mix, relative to the upstream sensor, by the time it gets to the downstream sensor.

It could mean you have a bad CAT, but you're not complaining about performance issues and you say it smells okay.

Downstream is the same as "second". So look for a leak after the first O2 sensor and before the second O2 sensor... namely the flex pipe.

Another issue could be too much fuel for some reason, like a fault in the fuel injection system someplace, but again you don't complain about performance.

It might just be like on my Kia Sedona: I changed the spark plugs, now I can't get the P0420 to go out. It runs perfectly and returns 50% better mpg than EPA estimates, so I enjoy the pretty light on my dash. But then I live in a "no emissions" state. It's a Wild West for auto care down here.
 

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Personally, I would try the non fouler/anti fouler trick before buying anything. Space the downstream sensor. It tricks the car into thinking its emitting less emissions than it actually is, which would result in no code.

I have seen this code many times on many cars, and none were because the o2 went bad or the injectors. I've done this trick to 3 different make/models to help friends pass emissions for cheap. I'm winning 3 of 3.
 

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Discussion Starter · #6 ·
Well, I get the same amount of mpg than before the code... no difference. I don't notice it bogging down as if the cat is clogged, flex pipe leakage is a possibility of course... I am due for my transmission service and my yearly engine tune - up which will address the possible issue with the P0300 code. I've done some research and more times than not, it is recommended to address the first code (P0300) and then the second code (P0420). So, I will get the service done and if the CIL comes back on I will address it from there. Thanks all for your advice... truly appreciated.
 

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Discussion Starter · #7 ·
Come to think of it, I had the fuel system maintenance done (injection into the fuel system) done when I got my breaks done back in August/September. Could this be an after effect of this? Like the fuel filter getting clogged or something similar?
 

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hello, i have a p0300 code which is a random misfire but the car continues to misfireand even sometimes stallalso you can hear the engine misfire loudly. hope you can help ty in advance
 

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Hello,

Looking for some advice on the above codes. My Elantra's check engine light came on while driving on the freeway. The codes state (from Auto Zone's diagnosis) that this could be several issues. I currently am scheduled for a transmission service and engine tune up. Anyone have experience with these codes and the common reason they come up? I will say that I cannot detect a misfire or smoke of any color coming from the exhaust...


Thanks!
It could be the catalytic converter being inefficient to work properly. the tricky part is to identifying which converter is malfunctioning. My instinct points to upstream converter located next to exhaust manifold. However, downstream converter could also be a strong candidate for P0420 engine codes.
 

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The P0420 code means the two O2 sensors (one upstream of the cat and one downstream of the cat) have very similar readings
This can be caused by cylinder misfire and hence the P0300 code.

If you have access to a scanner then you can see the readings of the up stream O2 and down stream O2 sensors
Check to see if their readings are similar. The up stream O2 sensor should be fluctuating and the down stream sensor should be relatively steady.

Bad O2 sensor can cause a P0420 code
But also having a P0300 code probably means the misfire is causing and/or adding to the problem
Solve the misfire problem first and look for any wiring harness problems around the O2 sensors

Good luck
 
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