2003 wrx code P0420
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This is a discussion on 2003 wrx code P0420 within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; hi all. i'm new to subaru vehicles and my 2003 wrx wagon is my first. i got it 3 days ...

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    2003 wrx code P0420

    hi all. i'm new to subaru vehicles and my 2003 wrx wagon is my first. i got it 3 days ago and this morning i got code P0420. i noticed a bit of a rough idle since i test drove it, but i figured it was something minor that a sparkplug change would fix. the car has around 85,300 miles. from what i can tell the muffler is a flowmaster with a magnaflow tip, but everything before that is stock (i may be wrong). i got it scanned at autozone.

    i'm posting a pic of the engine bay to see if you guys can spot any culprits.



    i hope its nothing major . thanks for your help.

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    Registered User EvoEatr's Avatar
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    P0420 is Catalyst system efficency below threshold....I would first make sure you dont have an aftermarket catless up pipe or downpipe..that would easily throw that code....also its rare but a bad/lazy 02 sensor can throw this code....first find out if you have all the stock Cat. Conv.'s on the car and well go from there...
    Last edited by EvoEatr; 02-28-2009 at 05:04 PM.

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    thank you for the reply EvoEater.



    that's one cat i believe, right after the turbo.

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    Moderator T0rque's Avatar
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    actually its two but whos counting... You may have an aftermarket up pipe throwing the code, could be o2 sensors with the mileage, or your cats could of simply failed. Good thing is catalytic converters are federally warrantied for 80k, since your close to that mark I would call your local subaru dealer and ask wassup.


    Lead Wrench @ WTF Tuning, LLC

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    so will it be ok if i drive it around like that for a while? also, if it is an 02, which one should i get and where should i buy it?

    thanks for the help guys.

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    Registered User EvoEatr's Avatar
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    That does look like the stock DP...If you can..check the up-pipe and the rest of the exhaust for signs of tampering...Yes you will be fine running around like this....your fuel mileage/drivability might suffer a little..but nothing crazy...If you do happen to need an 02 sensor....OEM...I have seen countless aftermarket sensors from NAPA and such actually cause problems....Subaru's are very touchy when it comes to some aftermarket componants...02 sensors being one of them....Also you could try clearing the code and see if it comes back...worth a shot....

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    Admiral Ackbar the 1st mycologist's Avatar
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    This car might have been partially reverted to stock without getting an appropriate tune. I'd put a DP on it and get a stage II map. Is that the stock intercooler?
    "From a little spark may burst a mighty flame." - Dante
    "The stitch is lost unless the thread is knotted." - Italian proverb

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    thats the intercooler that came with it, but i don't think its stock. i've noticed that it will go through a quarter tank real quick now. one other thing, when the light came on, a cold front came in. the previous days had been warm. thanks for the help guys.

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    Moderator T0rque's Avatar
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    Id personally replace both o2s. Id get an up pipe and do the EGR mod to get rid of that nonsense as well. Only buy the factory subaru o2 sensors.


    Lead Wrench @ WTF Tuning, LLC

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    Moderator Donkey's Avatar
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    Quote Originally Posted by LPPTundWRX View Post
    Id personally replace both o2s. Id get an up pipe and do the EGR mod to get rid of that nonsense as well. Only buy the factory subaru o2 sensors.
    I agree with replacing both sensors but do not buy from the dealership unless you have too.NTK or Denso sensors from Autozone will work just fine as they are what come on the car.Just don't buy the universal ones,buy the OEM fit with the connector.
    Resident Adviser @ WTF Tuning, LLC
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    will bosch O2 sensors do the trick? i think i read somewhere that all front O2s are made by denso, just rebadged for other companies. front denso is $226 and bosh is $195 from autozone. only bosch rear is available. will i be ok with bosch?
    thanks

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    Registered User EvoEatr's Avatar
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    My God...Those prices...you might be able to get ones from the dealership for cheaper...

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    update* i replaced the rear O2 and the CEL is gonne! i'm getting improved gas mileage and the performance is much better. i'm going to replace the front one as well for preventive maintenance.

    thanks everyone for the help, i really appreciate it.

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    I know this thread is a couple years old, but I am getting that code on my 03' Wagon as well. I just reset the ECU last night so I will see if it goes away. My 02 sensors were replaced a couple thousand miles ago. I have gone about 30 miles so far and it has not come back on so I will wait a couple days and see.

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    the only way i got rid of that code was with an open source tune. i installed a turbo-back exhaust and got tuned for "stage 2." i had previously replaced both sensors and did many resets to no avail. i have put almost 30K miles on the car since then and have had no issues.

    good luck

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