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Guest Message by DevFuse

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2000 Legacy- Replacing Catalytic Converter-do I need Sensors?

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8 replies to this topic

#1 wphamren



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Posted 07 April 2013 - 06:10 AM

A month ago, I finished a head gasket repair and other much needed maintenance on my 2000 Legacy. I then got the P0420 code from CEL; there are no other codes.  I went through all the online troubleshooting techniques with no success, so, I took it to my local dealer to do a catalytic converter efficiency test. They told me my cat is bad, but both sensors are O.K ( I trust it). So, can I just replace the cat, or do I need to replace the sensors because it will be a new cat? It will be a BOSAL cat, if that matters.

#2 ocei77


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Posted 07 April 2013 - 07:35 AM

90% of the time a 420 is caused by a bad front sensor.

I would google this code specifically for Subaru and go through those checks first.

Cats can go bad, but usually they last they life of the car.

If it was me, I'd replace the front sensor.



#3 johnceggleston


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Posted 07 April 2013 - 10:35 AM

what he said.


lots of things can cause a p0420 code. cats are low on the list.

but some shops and most dealers like selling new cats for $700? to folks who do not know better.



read thru this:



#4 1 Lucky Texan

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Posted 07 April 2013 - 10:45 AM

If they did the propane test I've read about, that should be conclusive.


But, as said, many people have been sold cats, when they just had exhaust leaks or bad sensors or other less expensive problems.

#5 wphamren



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Posted 07 April 2013 - 06:47 PM

I thank everyone who responded with the check first, but the catalytic efficiency check is done on the dealer computer to confirm/check if it is sensors or the cat. It's the cat. I still am wondering if the sensors need to be replaced when replacing the cat?

#6 johnceggleston


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Posted 07 April 2013 - 07:37 PM

if they replace the cats, they will likely replace the sensors too.

and it is likely that this will correct the problem.


but if you just replace the sensors now, and then wait and see if that corrects the problem, what have you lost?

you can always go back and repalce the cats.


and i doubt very seriously that they will give you a price break on doing both jobs at once.

they rarely do. ther is a book they go by that quotes the time for each job.



regarding the question, do the o2 sensors need replacing?

how many miles?

they do not last forever.

so at some point they need to be swapped.

150k is a long time for an o2 sensor.


exhaust leaks can also cause a p0420 code.

are the exhaust manifold gaskets in good condition?

was the exhaust disassembled during the HG job?

did the dealer do the HG job?

#7 wphamren



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Posted 08 April 2013 - 04:53 AM

John, appreciate the insight. Legacy is at 117000. It has not been my primary dirver until recently, so, it has not had much use the past couple years. I did have the exhaust manifold gaskets replaced during HG job. Dealer did not do job, but very reputable Subaru service place did. I will be doing the work myself on exhaust; I would have done HG if I had more time.


I see the point of replacing the O2 sensors first, and cheking for exhaust leaks, but doesn't a catalytic converter effiiency check determine that? That was the understanding I was given of the test. This was done at a Subaru dealer.

#8 idosubaru


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Posted 08 April 2013 - 06:55 AM

Subaru catalytic converters almost never fail.  the P0420 code is a debacle of a code, benign, and nearly worthless.  unless you want to do some extensive reading (http://www.subaruout...p0420-diag.html is the only in depth trained technician write up i've seen online), it's best to follow the advice of folks with experience verses the dealer.  dealers don't often do extensive testing, regardless of what they've told you, how they explain it, or how definitive they make their test sound.  maybe they did do extensive testing but usually they don't so it's hard for us to put much weight into that diagnosis knowing how they normally operate. you seem to hold the dealer in high regard, but they generally replace catalytic converters far more than necessary.  and i'm not faulting them for that - proper diagnosis of this code is a mess...which means time and money if you're paying the bill, so they gotta tough job balancing customer satisfaction.  replacing is the simplest and easiest solution even if it's not the most proper.


a $5 spacer will also get rid of the code.  you'll pass maryland emissions sniff tests just fine (i'm from Maryland).


replacing the converter is probably the same fix as the $5 spacer.  it's a band aid. both will work and are likely covering up some other small issue (read the link posted above).


if something bad enough caused the code, then it could come back in a few years with your new converter...so replacing isn't the high road it may seem.

Edited by grossgary, 08 April 2013 - 06:58 AM.

#9 wphamren



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Posted 08 April 2013 - 07:23 PM

Gossgary, appeciate the response. I will read the link you provided, and see what I can do to find the fault. I'm not worried about anything more than keeping my Subaru going in the best way it can. I will say, your expalnation is well received. I'm posting here because I'm not convinced by what the dealer told me, and the internet is FULL of reasons why the P0420 code is potentially so much more than the cat. I guess my best course is to go back to why this happened after the HG repair- starting with how it was reassembled. Thanks.

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