P0420 - Subaru Impreza GC8 & RS Forum & Community: RS25.com
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Old 05-28-2019, 08:47 AM   #1
Car: 2005 Impreza 2.5RS Wagon
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Join Date: Nov 2015
Location: Montreal Canada
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Posts: 3
Default P0420

Long story short about 2.5 years ago I got a code for a failing upstream sensor, I changed that and the code went away. About a year after I started getting a P0420 code only when I go on highways for about 10m+ (I work 5m away from home so I don't normally get the code). I got my head gaskets changed about 8-9 months ago. but the P0420 still comes back.

I have horrid mileage ever since I started getting a P0420.
I'm about to buy a new cat from Walker #54537 and a new downstream sensor DENSO #2344732. I already went to my local exhaust shop and other than having to replace some exhaust parts after the cat I have no leaks (at least from exhaust headers to cat I have none)

My question is could a leaking head gasket kill my cat or my upstream/downstream sensor. And would running rich cause soot on my exhaust tip and more than usual condensation out of my exhaust during winter ?
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Old 05-29-2019, 10:57 AM   #2
Car: 2006, 2.5i Black Wagon
Join Date: Apr 2015
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Posts: 9

Interesting... When I got that code I had a hole in the catalyst material.. and so did a friend of mine. Sounds like your having the same issue since it triggers on longer drives.
When my head gasket leaked it didn't effect the cat any. Granted it was an external oil leak... I wonder if the back sensor is getting coated from a possible hole and causing the fuel trim to be off.
Out of curiosity... does the car feel down on power any?
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Old 06-05-2019, 04:26 AM   #3
Samurai Jack
Car: 2002 WRX
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Join Date: Apr 2011
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Posts: 29

There is just not enough information to even guess where your issue might be coming from.

Couple of questions just to get started:
1. Last tune-up?
2. Any misfiring?
3. Running all OEM exhaust sensors?
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