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As I'm sure many of you know there are millions upon millions of P0420 threads. Many of them because of exhaust mods.

Anyway I did my searching and the only thread I found satisfactory was this one. P0420 Catalyst System Efficiency Below Threshold (Bank 1)?! - Subaru WRX Forum: WRX Forums

As I was driving to work today my cruise control stopped working, the check engine and tcs off light came on. The "cruise" light now blinks steadily at all times. I took it to o'rileys and used the code reader. I got "P0420 Catalyst System efficiency below threshold (bank 1)". There are no mods on the car currently. Should I do like the thread suggested and replace the rear o2 sensor? Car is an 08 wrx.
Ideas?
 

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Check the emissions control warranty for your car. Not the 36K bumper to bumper nor the 60K drivetrain, but the emissions control warranty specifically. It may still cover you. It will list what components are covered. If you're under that warranty, INSIST the car is repaired for free and if it is not get it in writing and contact EPA.
 

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Check the emissions control warranty for your car. Not the 36K bumper to bumper nor the 60K drivetrain, but the emissions control warranty specifically. It may still cover you. It will list what components are covered. If you're under that warranty, INSIST the car is repaired for free and if it is not get it in writing and contact EPA.
How do I look up the emissions warranty? The car is also prior salvage, I didn't buy it new. Would it still be covered?
 

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How do I look up the emissions warranty? The car is also prior salvage, I didn't buy it new. Would it still be covered?
It's in the owner's manual or a separate little booklet in the same pouch. For salvage, I don't know, sorry.
 

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It's in the owner's manual or a separate little booklet in the same pouch. For salvage, I don't know, sorry.
Unfortunately these days it seems the first thing someone does is throw out the owners manual. Its something that really boggles my mind. I've never bought a car that came with the owners manual.
 

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Argh, sorry. OK well first things first, check the wiring. Since the car is unmodified it could be a bona fide fault code from the sensor, so think wiring - sensor in that order.
 

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Argh, sorry. OK well first things first, check the wiring. Since the car is unmodified it could be a bona fide fault code from the sensor, so think wiring - sensor in that order.
:( Oh boy....I wish This happened a year ago. I no longer have the time or space to work on my own car. Is there a general place where I should start looking at the wiring? I'm guessing nearest to the sensor would be a good place to start? I'm ashamed to say though that I know nothing about the car. When I'm not at work I'm studying.
 

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It's either the sensor or there is an issue with the catalytic convertor.
 

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Yes follow the wiring starting backwards at the sensor and checking for frays etc. otherwise suspect the sensor itself, in which I have less faith than the cat because the sensor is electric whereas the cat is a metallic mesh. Suspect the cat itself only last I think.
 

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We had a thread a long time ago where one clogged somehow. Don't know your car history but at four years for a cat to fail would be discouraging. Not impossible but I hope improbable.
 

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Discussion Starter #12
Is there a "how to" for replacing the rear o2 sensor around here somewhere? I saw one for the front sensor but nothing on the rear.
 

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It doesn't get more basic then just unpluging it and unscrewing it. Look at the service manual..........
 

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It doesn't get more basic then just unpluging it and unscrewing it. Look at the service manual..........
Well I know that much lol (and forgot I had the service manual). I was referring to the location of the sensor on the exhaust. Also is there a way to test the sensor to make sure its bad? A new one is $125......don't feel like spending that much for nothing.
 

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The service manual shows you a picture of exactly where it is. There are only two. One before the turbo and one after. By merely looking under the car you would see exactly where the sensor is located. The service manual will give the full troubleshooting "tree" for a P420 DTC. It is located in the OBDII section. Have you even tried looking through the manual at all?
 

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The service manual shows you a picture of exactly where it is. There are only two. One before the turbo and one after. By merely looking under the car you would see exactly where the sensor is located. The service manual will give the full troubleshooting "tree" for a P420 DTC. It is located in the OBDII section. Have you even tried looking through the manual at all?
lol yea. There's so much stuff to go through. I looked through it for 3 hours and found nothing. I'm probably going to end up taking to a shop. Really don't want to but like I said I have no time for this :(
 

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Seriously.......It took me 5 minutes to find it. Go to en (H4DOTC)>Engine Diagnostics>Diagnostic Procedure with DTC>P0420. The rear O2 sensor is located in the 3rd (last) cat in the system. You would have immediately seen it by jacking up the car and looking.
 
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