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Old 06-05-2011, 07:12 PM   #1
lilstahlly22
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Member#: 265677
Join Date: Dec 2010
Location: Plymouth, MN
Vehicle:
2004 WRX/STI
Blue

Default P0420 Cel

Have a completely stock 2004 STi. Got this CEL today. Had it checked out at Checker Auto. People have said on here that a new cat has fixed it, and others say that a new 02 sensor fixed it. What do you all think? New TBE is going on in a few days so the cat wouldn't matter but if it's the O2 sensor I need to order one asap. Also, if I stay out of boost will it hurt the car to drive it? thanks for the help and yes I searched just couldn't find a definite answer!
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Old 06-05-2011, 10:03 PM   #2
lilstahlly22
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Join Date: Dec 2010
Location: Plymouth, MN
Vehicle:
2004 WRX/STI
Blue

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Anybody?
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Old 06-05-2011, 10:38 PM   #3
whiteimpreza1
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Member#: 259680
Join Date: Oct 2010
Chapter/Region: NESIC
Location: Northwest, CT
Vehicle:
06 2.5i impreza
Aspen white.

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i threw this code on my 06 2.5i. the dealer that sold me the car said it's from a bad cat, or the cat needs to be cleaned. They said we'll clear it, and if it comes back we'll get a new cat. So they cleared it and it's been probably 5 monthts and haven't seen it since.
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Old 06-06-2011, 01:57 PM   #4
AliBenn
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Member#: 487
Join Date: Nov 1999
Chapter/Region: TXIC
Location: Greatwood/Sgrlnd
Vehicle:
1998 /2005 STunIcorn
Acadia Green RS-T

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does this code come on when your are cruising at low rpm?(not hard on the throttle, jack rabbit starts and such)
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Old 06-07-2011, 03:39 PM   #5
lilstahlly22
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Member#: 265677
Join Date: Dec 2010
Location: Plymouth, MN
Vehicle:
2004 WRX/STI
Blue

Default

its on all the time now. and I ordered a new o2 Sensor and will install today to see if that fixes it. if not, my new tbe is going in next week so that would help the cat anyway!
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Old 06-08-2011, 11:27 PM   #6
danny_d19
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Member#: 180748
Join Date: May 2008
Chapter/Region: VIC
Location: BC
Vehicle:
2005 STi
CGM

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Had this CEL come on in my 05 STi a bunch of times, in addition with not being able to boost past 8-9 psi. Turned out to be a clogged cat. Put on a new 3" downpipe and it fixed it
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Old 06-08-2011, 11:32 PM   #7
mikeydrives1
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Member#: 262117
Join Date: Oct 2010
Chapter/Region: Tri-State
Vehicle:
2001 Forester
STi II type M

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This code is common from what i read among here, Forester.org, Outback.org and RS25.... Simplest fix is a spark plug fouler which pulls the secondary 02 sensor away from the gas stream, thus reading less emmisions.

For whatever reason or another, it seems common (including my own car). The cat isnt always going bad. My car threw this even with a NEW 02 sensor and cat... it popped up twice on a 500 mile roadtrip (crusing at low RPM for long distances) after the fact that it was replaced , so perhaps its just a bug, at least among older subarus.
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Old 06-09-2011, 02:01 AM   #8
Buffman
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Member#: 189212
Join Date: Sep 2008
Location: MI
Vehicle:
2002 WRX wagon
Silver

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Mine is on because of the single cat Downpipe. It would go on and off, and figured maybe I pinched an 02 wire, so I replaced it, and it's sitll on. I figured I might run into it with an aftermarket cat in a DP, so I'll probably just have to have it tuned out, as there's no room for a spark plug fouler.
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Old 06-09-2011, 09:53 AM   #9
AliBenn
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Location: Greatwood/Sgrlnd
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1998 /2005 STunIcorn
Acadia Green RS-T

Default

meh.

mine comes on when i cruise at 60-70mph low RPM for extended period of time

never comes on during city driving(change in RPM's)

i dont have a rear O2
have a AP that doesnt even read for this code!

i just clr the code after every long road trip i take.
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