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Old 01-09-2017, 03:16 PM   #1
Arkadion
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Member#: 358368
Join Date: Jun 2013
Chapter/Region: MWSOC
Location: Houghton, MI
Vehicle:
2005 Impreza 2.5RS
Crystal Grey Metallic

Default Another damn P0420 thread

What are my options for knocking out a P0420 so I can pass emissions?

My '05 Impreza RS with over 200k miles has developed a persistent P0420 code. I've had it inspected by a shop, and they've determined the cats are definitely shot. I can erase the code, but it'll come back within 100 miles.

Relevant mods are a williaty hybrid intake and a 2.25" OBX catback. Intake has a brand new OEM paper filter.

I need to get the code to go away permanently before March when it's due for emissions testing in Illinois. I only have to pass the plug-in test. I've already failed once because of too many not-ready states after trying to erase the code before getting tested.

I have no other codes. The car runs fine when the CEL comes on, other than losing cruise control.

I understand the '05 Impreza uses the rear O2 sensor to adjust fueling on top of checking the cat efficiency, so I can't use an anti-fouler or a spacer. Though if anyone has physical proof from the ecu that it does, that'd be great since nobody believes me outside of the N/A Subaru community.

I was looking at Hi-flow cats, but those don't appear to guarantee a solution. Subaru wants about $1100 for a new one, and the local shop estimated $1000 to make and install new a new pipe. Are there any recommendations for regular OE replacement manufacturers? OE or heavy metal? I know Stromung has a well-loved cat from searching around on here.

I'm also considering tuning out the problem, since I have the tune-able ECU and an experienced friend that can do tuning. He said he could force ready the systems or possibly tune it to run catless, but I'm not sure about how to deal with the O2 sensor affecting fueling. I'm not sure I want the noise and rasp that comes with catless, either.
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Old 01-09-2017, 03:33 PM   #2
v2rocket_aka944
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Member#: 408701
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Default

"Hi-flow cats" are pretty much marketing BS left over from the 1980s.
Modern cats really aren't any restriction to power/flow on a non-turbo car unless they are clogged (which isn't the cat's fault).

Walker exhaust is a good reliable brand and they make a "bolt up direct replacement" for your car for under $250...

walker 54573
https://www.amazon.com/Walker-54573-.../dp/B001AGPD96
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Old 01-09-2017, 04:19 PM   #3
Arkadion
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Member#: 358368
Join Date: Jun 2013
Chapter/Region: MWSOC
Location: Houghton, MI
Vehicle:
2005 Impreza 2.5RS
Crystal Grey Metallic

Default

That looks like it should be a good solution, and right around $250 with all new hardware.

Is the second chamber on the cat pipe a resonator? I did tell the shop it had two cats on the pipe, but I don't think I ever checked.
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Old 01-09-2017, 04:25 PM   #4
v2rocket_aka944
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should be a 2nd cat.
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