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Old 03-07-2011, 05:35 PM   #1
averagebob
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Angry Check engine, autozone says P0420 halp?

Ok 01 sub legacy L 2.5 NA

Mods
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Recently replaced (within 2 months)
Upstream 02 sensor



So the code is (Catalyst efficiency below threshold - bank 1)

1. Rich A\F ratio
2. Ignition system malfunction
3. Faulty Catalytic converter

So, wtf? I was just driving down the road after pulling out of a parking lot and my check engine light popped on, I hadn't floored it or anything just slowly pulled out like normal driving. wtfbbqs


Oh yeah, I have 400$'s what should I get\do? So far I have rims on my list to get because mine are fubar.
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Old 03-07-2011, 05:59 PM   #2
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Its probably from the O2 sensor you just replaced. Did it have the same amount of wires as the one you pulled off of it? I know when i was buying O2 sensors they offered both 3 and 4 wires for extra confusion. The O2 precat was 3 and post cat was 4 for me (99 RS)
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Old 03-07-2011, 06:05 PM   #3
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^ no it fit perfectly, IIRC 3 wires, same on same off and its been on almost 2 months so I figured the down stream o2 went out (because the upstream had just died).
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Old 03-07-2011, 06:27 PM   #4
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ttt
asdf;lkj
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Old 03-07-2011, 06:39 PM   #5
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P0420 is your cat pipe O2 sensor.
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Old 03-07-2011, 06:53 PM   #6
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So its the downstream one like I thought? I replaced the one closest to the engine cause that's the p203 or whatever code. You think its the further o2 sensor?
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Old 03-07-2011, 07:09 PM   #7
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a P0420 9 times out of 10 will be a bad cat converter. Yes the upstream a/f sensor can also cause this same condition. The downstream o2 will not cause this code because it is after the cat and it doesn't read for that.
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Old 03-07-2011, 07:17 PM   #8
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Hmmm could the autozone replacement UPstream o2 sensor have caused my cat to fail? I heard those are fugging expensive?
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Old 03-07-2011, 07:17 PM   #9
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if the upstream 02 sensor was bad before, it could be damaging your cat in the meantime. Then the cat fails and errors out your post-cat sensor. Stromung high-flow cat upgrade? Sometimes higher flow aftermarket cats are cheaper than OEM... just sayin
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Old 03-07-2011, 07:20 PM   #10
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Quote:
Originally Posted by blackfang View Post
a P0420 9 times out of 10 will be a bad cat.
That's correct

Quote:
Originally Posted by blackfang View Post
The downstream o2 will not cause this code because it is after the cat and it doesn't read for that..
Thatís wrong, the rear o2 along with the front checks the efficiency of the cat.

Subaru cats suck and are expensive. It most likely your cats are bad but there's always the chance its your rear o2. Be careful with aftermarket cats because all cats are not created equally. Factory cats will set you back around $700.00
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Old 03-07-2011, 08:25 PM   #11
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700$ what the ****! Could I clean the one on my car with gasoline or acetone? Like could it be build up? hrmmmm aftermarket cat that fits stock pipe is looking good, any prefered sites of where to buy?

that sucks balls
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Old 03-07-2011, 08:36 PM   #12
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whats the mileage?
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Old 03-07-2011, 08:38 PM   #13
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141xxx
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Old 03-07-2011, 08:38 PM   #14
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Quote:
Originally Posted by averagebob View Post
700$ what the ****! Could I clean the one on my car with gasoline or acetone? Like could it be build up? hrmmmm aftermarket cat that fits stock pipe is looking good, any prefered sites of where to buy?

that sucks balls
A cat that has overheated can't be cleaned. It literally melts into a valcanic rock-like blob that plugs it up and prevents flow.

http://www.boxer4racing.com/exhaustw.htm
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Old 03-07-2011, 08:42 PM   #15
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get a high flow cat and then spark plug non fouler on the second o2 sensor.
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Old 03-07-2011, 08:42 PM   #16
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Quote:
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141xxx
from my experience, all subarus over 140k have the check engine light on and it sends weird codes, but i may be mistaken
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Old 03-07-2011, 08:44 PM   #17
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^ Reset the computer\clear code and see if it happens again?^

I was down there to change my upstream and it didn't sound like it was broken up and I get great gas mileage, literally happened this afternoon (the error). I doubt it melted in to a blob. ty for the link!

cheapest one is 600$'s? wholly but****. . . . I think I'll try to clean it before I spend that much damn so much for getting ahead with these orders.
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Old 03-07-2011, 08:53 PM   #18
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Quote:
Originally Posted by Burnout View Post
That's correct



That’s wrong, the rear o2 along with the front checks the efficiency of the cat.

Subaru cats suck and are expensive. It most likely your cats are bad but there's always the chance its your rear o2. Be careful with aftermarket cats because all cats are not created equally. Factory cats will set you back around $700.00
Yes I am quite familiar with what it does. However a bad rear O2 in 95% of the cases will not set a p0420 code unless the front A/F is also bad. The rear O2's typically set a P0137/38 DTC. A front A/F sensor has more of a fluctuating wavepath type reading where the rear does not. The rear O2 has more of a steady reading. However, the front A/F can go bad and become "lazy" causing the p0420 code. That is why Subaru changed their way of diagnosing in a P0420 code in '08 and now it requires much more of a thorough diagnosis that requires driving, pulling freeze frame data, and catalyst readiness tests as well.

Yes Subaru cats are expensive and so are the PZEV ones. I have seen those bad boys hit almost $2,000.
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Old 03-07-2011, 08:58 PM   #19
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Quote:
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Hmmm could the autozone replacement UPstream o2 sensor have caused my cat to fail? I heard those are fugging expensive?
There is always a possibility if that previous sensor was bad. It sure can cause the cat to fail. If the sensor is lazy and not reading correctly, the ECM could be dumping more fuel than what is needed and cause the cat to go bad.

** Also be careful of aftermarket high flow cats. A nice alternative, but because they work, doesn't mean they will not cause a P0420 code to come back. I have seen it plenty of times where the cat is installed and a the light comes back on. It just is not up to the efficiency level of what the ECM and what the car is programmed for.
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Old 03-08-2011, 01:20 AM   #20
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Quote:
Originally Posted by blackfang View Post
a P0420 9 times out of 10 will be a bad cat converter. Yes the upstream a/f sensor can also cause this same condition. The downstream o2 will not cause this code because it is after the cat and it doesn't read for that.
9/10 times will be a bad O2 sensor. 1/10 will be a converter.
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Old 03-08-2011, 06:13 AM   #21
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9/10 times will be a bad O2 sensor. 1/10 will be a converter.
If you think so.
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Old 03-08-2011, 07:41 AM   #22
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so I could try the rear 02, its only like 60$'s if that doesn't work then I could get the cat... but wholly **** its soooo expensive , and bank 1 is running rich how would that happen from a messed up cat?

Hrmmm i assume long term driving with this issue would be a bad idea but this week I have no choice I'm in ohio on orders so I'm sol for the next 4 days and a 300mile trip back home.

I think I'll pull the cat at home, if its not a blob I'll clean it with gas\acetone and try again, still no go I'll get a new rear o2 sensor\reset the comp.

Thanks guys\gals for the help.
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Old 03-08-2011, 08:13 AM   #23
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you could try to hook it up to a decent scan tool and see what the downstream 02 sensor is reading, it should fluctuate like that of the primary, but not as much. in my experience the o2 sensor probably needs replaced, but check it on a good scan tool, throwing parts at things sucks
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Old 03-08-2011, 11:52 AM   #24
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^ indeed it does, the scan tool from autozone wont give readings for things only read error codes. I want to get one of those blue tooth obd2 readers and torque for my droid that will give A\F ratios but in the mean time I will clean the Cat and if that doesn't fix it I'll get a new o2 sensor =-\.
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Old 03-08-2011, 11:55 AM   #25
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http://www.discountconverter.com/dir...roductID=43445

200$'s For a cat and pipe, why when that other site had them for 600?
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