I have an 04 Legacy approaching 200K that I intend to keep as a commuter car. About six months ago it started throwing a P0420 that I would normally clear and keep driving. I suppose the car is made to comply with CA emissions standards - it has dual front catalytic converters and the error description explicitly mentiones 'bank 1'.
(http://www.justanswer.com/subaru/5qgxl-subaru-legacy-2004-subaru-legacy-35th-anniversary.html)

Lately some rattling appeared from the passenger side coverterter and the error will pop up almost immediately after I clear it. I took it to a mechanic who says that it's a bad cat and I was quoted at $900 for replacement of just the passenger side one. I declined and aggreed on a three-step flush as a first measure, which did not help. The question is - what would be the cheapest way to fix this?

I was able to find these parts online:

 

This seems to be the correct part but rather expensive: http://subarupartsforyou.com/cp_partdetail.php?partid=11285

Here's another option but no CA complience: http://www.converterwarehouse.com/catalytic_converters/converter_detail.cfm?productID=259778

And this is the cheapest one: http://www.jcwhitney.com/eastern-direct-fit-catalytic-converter-49-state-legal-no-ca-shipment/p3040064.jcwx?filterid=d1939y2004g633j1&skuId=4362464
 

Will the latter two cause any problems? Can I reuse the old sensors?

Thanks!

 

the P0420 code is a debacle.  it's not necessarily the converter that needs replaced, usually it's not.  converters typically last the life of the vehicle easily.  mechanics/shops often throw converters at this code obsessively because it's the easiest solution to what is a very tricky code to diagnose.

 

here's a mechanic who is very familiar with Subaru's that describes it and proper diagnosis for this code:

http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diagnosis.html

 

problem is few mechanics know how to do what he does or how to do it quickly and efficiently.

 

i'd first install a $5 spacer to get rid of the code.  they're all over ebay, i have a few on hand because the problem is common across all vehicles.

 

 

if something caused the converter to fail, it'll eventually cause the new one to fail.

 

some steps:

 

1.  repair any exhaust leaks first.

2.  get the engine in excellent running condition.  spark plugs, wires, air filter, fuel filter, run some seafoam and/or MMO through the intake, gas, oil.

 

rattles are usually benign and meaningless heat shields.  though they could possibly change temperature gradient and affect P0420 code triggering as I think GD said.

I tend to agree with the bad converter diagnosis. I was looking at the converters together with the machanic and I don't see the shields being loose anywhere. The rattle appeared after I've been sporadically getting and clearing the error already but the frequency increased dramatically after it started. Prior to that, however, I'd noticed that it usually happens after an oil change or after pumping gas. 

There are no ehaust leaks that can be detected, by a manual inspections at at least. 

I didn't think they went to the 2 cats in the Y-pipe until MY2005, but maybe this was some kinda special end of year 2004 thing. My understanding was the only difference in CA vs federal was the cat type (at least the first cat being 'better' on CA-spec cars) and the rear O2 sensor location (back of front cat in CA-spec vs on the 2nd cat in federal). Might be helpful to verify the exhaust config visually if you can.

 

+1 to what GG said, especially spark plugs, wires, air filter. How much $ are you looking to drop into a 200Kmi car? Aftermarket cats aren't really all that great quality wise, I've heard life expectancy can be as short as 2-3 years in rust belt states.

 

I would throw a spacer (spark plug anti-fouler) on the rear O2 and see if that helps with the CEL. It's easy, cheap, and will last for years with no negative effects. You can get them at Autozone/NAPA ("Help!" brand).

I didn't think they went to the 2 cats in the Y-pipe until MY2005, but maybe this was some kinda special end of year 2004 thing. My understanding was the only difference in CA vs federal was the cat type (at least the first cat being 'better' on CA-spec cars) and the rear O2 sensor location (back of front cat in CA-spec vs on the 2nd cat in federal).

The CA emissions 04 had 3 cats. One in each header line and a 3rd after the Y. Bank 1 is the passenger side. I would (and did) replace the front 02 sensor there.

 

O/

"rattle" - did the mechanic diagnose that?   i'd be checking the timing belt tensioner.  that has the new-style tensioner which is prone to failure.  they can start slapping and generally not holding tension on the belt.  i could envision that temporarily affecting timing and possibly being related to a P0420 code that's on the verge of tripping.   it would be noticed under the a/c compressor, behind the timing belt cover, generally they make noise more under load - accelerating. a mechanics stethoscope on the timing cover can help.

 

Subaru converters last the life of the vehicle, they do not fail, something causes them to fail - poor running, major engine catastrophe, etc.

 

Sure reuse your sensors, you'll get a code if they're really bad. or terrible gas mileage. if they're original they should be replaced, they're generally lazy well before 200,000. 

 

Impossible to say if you'll encounter problems with a given converter.  It's unlikely, but possible. Subaru converters generally have more of the expensive materials in them, that's why they cost more, so they're more robust.  Aftermarkets are cheaper, lesser expensive materials, with the idea they aren't likely to be used nearly as long as an original.  Usually doesn't matter, but it may depending how egregious the causes/P0420 issue was/is.  Knowing how the mechanic diagnosed a bad converter and if he identified a root cause of why it failed would be helpful to know. 

 

If the root cause is never identified then an aftermarket is going to be damaged quicker than a Subaru OEM.....will still take awhile though, but it's highly variable.

 

It would also be helpful to know the conditon of the vehicle/engine which you didn't answer the last time I asked. Plugs, wires, PCV valve, air filter.
MMO & Seafoam through the intake, gas, and/or oil has gotten rid of the P0420 code before, I would try it. 

It takes minutes and $5.