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Old 11-09-2020, 08:52 AM   #1
bombsquad41782
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Default Po420 2004 Subaru wrx

kinda stumped guys maybe I can get some input. I have a po420 code I had the code read and it said it was bank 1. I replaced bank 1 o2 sensor with a oem denso. After about 25 miles the light popped back on. The car is completely stock except for a grimmspeed up pipe. I do have a hole I noticed yesterday after the second 02 sensor. The car has a 149k for miles. Is it common for cat in the down pipe to go bad? Any help is appreciated
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Old 11-10-2020, 09:47 AM   #2
Samurai Jack
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Originally Posted by bombsquad41782 View Post
I do have a hole I noticed yesterday after the second 02 sensor. The car has a 149k for miles.
Are you saying you have a hole in the exhaust pipe or the cat?

Yes, Cats do go bad eventually. They are a service item and are not expected to last forever.
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Old 11-10-2020, 06:20 PM   #3
bombsquad41782
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I have a hole in the exhaust right after the cat with bank 2 02 sensor. If I replace the exhaust which I have ordered with a stock Walker. What is my option if the cat is plugged in the downpipe?
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Old 11-10-2020, 06:55 PM   #4
shagfagon
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Quote:
Originally Posted by bombsquad41782 View Post
kinda stumped guys maybe I can get some input. I have a po420 code I had the code read and it said it was bank 1. I replaced bank 1 o2 sensor with a oem denso. After about 25 miles the light popped back on. The car is completely stock except for a grimmspeed up pipe. I do have a hole I noticed yesterday after the second 02 sensor. The car has a 149k for miles. Is it common for cat in the down pipe to go bad? Any help is appreciated
Well P0420 is tripped when your post cat O2 sensor outputs readings outside of normal.

You changed your front o2 which does not output anything that has anything to do with P0420. Your front o2 is for closed loop fueling, nothing to do with catalyst efficiency, AKA P0420.

If your cat was plugged in the downpipe your car would run like ****.

Change your rear O2, or buy a tactrix and disable P0420 like the rest of us.
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Old 11-10-2020, 06:57 PM   #5
shagfagon
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Are you saying you have a hole in the exhaust pipe or the cat?
Since he is talking post rear o2, none of this matters even a little bit.
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Old 11-11-2020, 08:40 AM   #6
Samurai Jack
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Since he is talking post rear o2, none of this matters even a little bit.
Well, now that we know the hole is in the exhaust pipe after the cat, I agree.

His DP may not be plugged, but the cat could be starting to decay, which may be causing an issue.


Simply shutting off a CEL is never the correct answer UNLESS you know for certain the code is false.

How old is this DP you have installed?
The car may have 149K on it but what about the DP?
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Old 11-11-2020, 09:07 AM   #7
bombsquad41782
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The down pipe is as old as the car I believe I bought the car in 2016 the car was stock I had a uppipe installed by a Subaru mechanic. So I assume its as old as the car is
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Old 11-12-2020, 10:26 AM   #8
Samurai Jack
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The Grimmspeed UP is really nothing more than a pipe. There is no cat involved so no potential of a cat disintegrating.

But since you are talking an Rear O2 sensor, the UP has nothing to do with that.

I presume you have the stock DP.
As said before, YES, a cat can go bad and be causing your issues.

You can't tell by looking at the DP if the cat is bad or not.
You are going to have to remove the DP and inspect internally.
Grimmspeed makes very nice catted DPs.

I highly doubt the hole in the exhaust AFTER the cat / O2 Sensor is causing any issue since the O2 Sensor has already done it's job.
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Old 11-13-2020, 12:03 PM   #9
shagfagon
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Well, now that we know the hole is in the exhaust pipe after the cat, I agree.
He said this in his first post that the hole was after the rear o2 sensor. You could literally sawzall the exhaust off after the rear o2 sensor, it wouldn't matter.
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