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Guest Message by DevFuse

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2002 Outback P0420 code AFTER new CAT converter

Outback Catalytic converter oxygen sensor check engine code p0420

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9 replies to this topic

#1 thedoctor


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Posted 05 June 2014 - 07:58 AM

Mom's 2002 Outback keeps throwing the code p0420.   I'm told this means "catalytic converter". 


History (within the last six months): 2002 Outback, odometer 165,000, purchased from used Subaru sales/repair business.


- Engine bearing went out. Highway breakdown within 1000 miles of purchase.


- Short block replaced.


- Check engine p0420. Shop replaced cat saying maybe the engine failure ruined the cat.


- Check engine p0420. Shop says 2002 Outbacks have this problem, not 2001, not 2003. Shop says we could 1) try various fuel additives, 2) replace cat with genuine Subaru part for around $1000, or 3) replace cat with a high performance cat Magnaflow for $500.


- Installed Magnaflow


- Check engine p0420. Shop says again its just this year for some reason. Options: 1) replace magnaflow with Subaru cat, and receive $500 credit, 2) swap 2002 computer out with a 2001 Outback, since its really the computer that is having the problem, or 3) keep trying different additives.


When I mentioned o=o2 sensor, the shop said there's another code for that, so that isn't the problem.


So what to do? Is this 2002 Outback cat problem a well known problem? Because I'm not finding a lot of talk about it. How exactly does information about the catalytic converter flow to the computer? How are o2 sensors related?  How to get this check engine light to go away so my poor mother can drive in peace?

#2 zombieforce


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Posted 05 June 2014 - 08:28 AM

I would try a rear o2 sensor. If its reading slow it will see cat as inefficient and flag a 420 code. They need to be watchi g the data streamon a scanner and make sure the rear sensor is working. Its only job is to monitor the cat.

#3 john in KY

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Posted 05 June 2014 - 09:17 AM

Had the same code in my 95 wagon last month. Replaced both O2 sensors and no more code. 

#4 grossgary


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Posted 05 June 2014 - 09:43 AM

this is not a "2002" problem. 


best fix is to ignore it.  if you have emissions - clear it and see how long it goes before it comes back - you may be able to just clear, pass emissions, and do that ever time.


you can install a $5 spacer between the rear O2 sensor and the exhaust, that will often alleviate the code.  lots of info on web about it, they're readily available on amazon and ebay.


P0420 codes are very tricky to diagnose - which is why your mechanic is guessing and throwing the wrong parts at it.  nearly any component related to the engine can trip this code.  mechanics and dealers don't diagnose this code.  replacing the converter is not the proper fix, it may "cover it up" like a band aid for awhile, but that's not the actual fix...obviously in your case. converters last the life of the vehicle, they are amazingly robust on Subarus - regardless of all the backseat chatter like "maybe your converter is clogged" - wrong. that's a false notion widely perpetuated. installing a new Subaru converter is probably a bad idea.  the converters were probably never bad to begin with, but even if they were - whatever caused the others to fail, will eventually cause the Subaru one to fail again too.  the cause needs addressed.



P0420 is one of many things engine related (but not the converter itself). 


it would be good to know if it's running lean or rich, but there's no easy way to check that except - what kind of gas mileage are you getting


here are a few items that are good to have in good repair/condition anyway, so i generally go through these:


1.  exhaust leaks need fixed, particularly any in front of the converter

2.  seafoam the intake and fuel

3.  tune up - stock OEM NGK plugs, Subaru wires (or high quality wires), air filter, PCV valve

4.  adjust the valves - note any that are particularly tight

5.  replace O2 sensors - they could be lazy - but this is just a guess and shot in the dark - i figure i'm replacing them once during the 100,000+ miles i own the car anyway so i just replace them as preventative maintenance and to prevent lazy sensors/reduced gas mileage.

6.  some folks have commented that heat shields being missing from the exhaust can cause the code, since that interferes with temperature characteristics of the exhaust.


outside of that you need to connect to the OBDII data port and find out what is tripping this code - some parameter is out of spec.

#5 thedoctor


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Posted 05 June 2014 - 09:55 AM

OK. That's a good list to work on.


I think I need to purchase a device to connect to OBDII data port... one that will allow me to clear this code without disconnecting battery. Also one that will allow me to read o2 sensor data. 


Any recommendations on what device would serve the budget minded amature mechanic?

#6 1 Lucky Texan

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Posted 05 June 2014 - 10:24 AM

yeah, most of the simple definitions for the various DTCs that mention a car part or system, don't really mean "replace that part" - they just point to some sensor reading that is out of allowable range.

P0420 can have many sources and it rarely is a bad cat conv.

exhaust leaks,vacuum leaks, even bad wiring from rodent infestations, etc.

always best to start with tune-up and inspection type stuff.

I have an older Innova 3100 handheld scanner, but if you have a smartphone, or a laptop, there are devices/cables and programs that are very affordable - free.

Edited by 1 Lucky Texan, 05 June 2014 - 10:26 AM.

#7 zombieforce


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Posted 05 June 2014 - 11:11 AM

U scan will work with a smart phone think its about 69 dollars its decent. I used one a friend bought and other than it being slow it will clear your codes just fine. If theres not emissions testing I
where you live dont worry about the code. Tons of cars around here in east Tennessee with check engine light on for that very code.

#8 grossgary


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Posted 05 June 2014 - 12:46 PM

buy the cable and you can get free software, etc.




Diagnosing the data is tricky, would be nice to know how quickly your O2 sensor is responding and if it's running lean or rich and which cylinders.


I like the cheap ebay OBDII scanners just for reading/clearing codes. i don't have to worry about breaking them, grease, taking them on trips, etc.  break and replace.

Edited by grossgary, 05 June 2014 - 12:46 PM.

#9 zombieforce


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Posted 05 June 2014 - 03:06 PM

I have a genuine vag com cable will that work with free ssm?



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Posted 05 June 2014 - 04:37 PM

Gary, that thread says it'll only work with 99-09, do you know of one that would work like that for the 95-99 ecus

Also tagged with one or more of these keywords: Outback, Catalytic converter, oxygen sensor, check engine code p0420

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