p0420 code all stock 04 wrx minus a cbe - Subaru WRX Forum
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#1 Old 08-03-2013, 01:33 PM
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p0420 code all stock 04 wrx minus a cbe

i just got my 2004 wrx about two weeks ago i installed a non name brand cat back exhuast drove roughly a tank of gas through it no problem noticed i was getting bad mpg so i cleaned my maf and reset the ecu. when i turned the car back on there was a cel. i again reset the ecu and no light came on i drove it for about a week. 300 miles or so then the cel came back on it was a p0420 i reset the ecu and its been off for about 50 miles or so. im new to subaru and the autozone people are no help, could this hurt my car and what could be setting it off. everything else is bone stock. ive researched all wrx forums but most everyone else with this code has a up pipe or aftermarket dp is it something simple or am i endangering my car by still driving it daily?!Cry!
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#2 Old 08-03-2013, 02:50 PM
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The code is usually for an o2 sensor, but there's no 100% way to know if an o2 is your problem. Could be one of your cats as well. The o2 sensors are the place to start though as they're cheaper.

Keep your fingers crossed its not a cat! They're ridiculously expensive for your gen.


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#3 Old 08-03-2013, 03:51 PM
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Does the car drive any different than normal? More than likely it is the O2 sensor. If it's the front they run roughly $150 rear is usually around $80. Go with the least expensive fix first.

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#4 Old 08-03-2013, 04:09 PM
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A clogged cat can affect gas mileage as well. You're gonna have to take each piece that has a cat out and inspect.
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#5 Old 08-03-2013, 04:16 PM
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No it drives totally fine. And the cel doesn't come back on after 100 miles. And the front o2 sensor seems so expensive its just as much as a new up pipe. But yeah ill check out those options I wad thinking maybe just do the spark plug de fouler like I would for a catless application. Thanks for the responses as I'm sure this is a pretty common and highly asked question
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#6 Old 08-03-2013, 04:52 PM
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You can't do that. You need to fix whatever is wrong with the car. The front O2 sensor is mandatory for the operation of the car to be in proper working order. You don't touch that sensor when you go catless.

Last edited by brfatal; 08-03-2013 at 06:26 PM.
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#7 Old 08-03-2013, 05:15 PM
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Quote:
Originally Posted by brfatal View Post
You can't do that. You need to fix whatever is wrong with the car. The front O2 sensor, if that's what it is, is mandatory for the operation of the car to be in proper working order. You don't touch that sensor when you go catless.
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#8 Old 08-03-2013, 05:21 PM
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I'm glad Justin never looked under my hood on the blaze.
He'd have a sore hand and I'd have a beat sore ass... Hehehe
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#9 Old 08-03-2013, 05:33 PM
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Alright well I know aftermarket up pipes that are catless people have done that for the o2 in the cat but alright ill investigate thanks
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#10 Old 08-03-2013, 05:48 PM
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What do you do when you go with a catless up pipe though? Because I'm thinking about doing that next since the cat is right near the turbo
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#11 Old 08-03-2013, 06:24 PM
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Quote:
Originally Posted by 04silverwrx View Post
Alright well I know aftermarket up pipes that are catless people have done that for the o2 in the cat but alright ill investigate thanks
That's an EGT sensor, not your O2 sensor.

That being said, a front O2 sensor is not going to cause a cat inefficiency code. Either your rear sensor is dead or your cats are going bad.
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#12 Old 08-03-2013, 08:05 PM
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Alright thank you
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