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Best Selling Genuine Subaru Valve Cover Gaskets

  • We Stock the following top leading brands, including Nippon Reinz, Ishino Stone
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We stock Valve Cover Gasket parts for most Subaru models, including Baja, Crosstrek, Forester, GL, Impreza, Legacy, Loyale, Outback, SVX, WRX, WRX STI, XV Crosstrek.


Nippon Reinz
2004 Subaru Impreza Engine Valve Cover Gasket 4 Cyl 2.0L Nippon Reinz

P311-2EE285D    W0133-1634731  New

Qty:
15.57
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • Spark Plug Well Seals and Cam Plug 1/2-Moons Are Serviced Separately
  • Right
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC
2004 - Subaru Impreza H 4 Cyl 2.0L - 1994
Ishino Stone
2007 Subaru Forester Engine Valve Cover Gasket Ishino Stone

P311-27FAD58    W0133-1768192  New

Qty:
42.43
Ishino Stone Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • Spark plug well seals and cam plugs are not included (order separately).
  • Right
Brand: Ishino Stone
Additional Fitment Information:
Vehicle Aspiration
2007 - Subaru Forester Turbocharged
Nippon Reinz
1998 Subaru Legacy Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-50C2643    W0133-1635834  New

Qty:
14.48
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Eng: 993723-047282, Production: 07/01/1997-09/30/1997
  • Spark plug well seals and cam plug ½-moons are not included (order separately).
  • Left
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Engine Range Prod. Date Range
1998 - Subaru Legacy H 4 Cyl 2.5L - 2458 993723 and up 047282 Fr:07-01-97 To:09-30-97
Nippon Reinz
1996 Subaru Legacy Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-50C2643    W0133-1635834  New

Qty:
14.48
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Production: 06/1996-
  • Spark plug well seals and cam plug ½-moons are not included (order separately).
  • Left
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Prod. Date Range
1996 - Subaru Legacy H 4 Cyl 2.5L - 2458 Fr:06-00-96
Nippon Reinz
1997 Subaru Legacy Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-50C2643    W0133-1635834  New

Qty:
14.48
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Production: 06/1996-05/1997
  • Spark plug well seals and cam plug ½-moons are not included (order separately).
  • Left
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Prod. Date Range
1997 - Subaru Legacy H 4 Cyl 2.5L - 2458 Fr:06-00-96 To:05-00-97
Nippon Reinz
1997 Subaru Legacy Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-50C2643    W0133-1635834  New

Qty:
14.48
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Eng: -989330, Production: 06/1997-06/1997
  • Spark plug well seals and cam plug ½-moons are not included (order separately).
  • Left
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Engine Range Prod. Date Range
1997 - Subaru Legacy H 4 Cyl 2.5L - 2458 up to 989330 Fr:06-00-97 To:06-00-97
Nippon Reinz
1997 Subaru Legacy Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-50C2643    W0133-1635834  New

Qty:
14.48
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Eng: 993723-047282, Production: 07/1997-09/1997
  • Spark plug well seals and cam plug ½-moons are not included (order separately).
  • Left
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Engine Range Prod. Date Range
1997 - Subaru Legacy H 4 Cyl 2.5L - 2458 993723 and up 047282 Fr:07-00-97 To:09-00-97
Nippon Reinz
2004 Subaru Impreza Engine Valve Cover Gasket 4 Cyl 2.0L Nippon Reinz

P311-5543F4B    W0133-1653912  New

Qty:
36.89
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • Spark Plug Well Seals and Cam Plug 1/2-Moons Are Serviced Separately
  • Left
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC
2004 - Subaru Impreza H 4 Cyl 2.0L - 1994
Ishino Stone
2004 Subaru Forester Engine Valve Cover Gasket Ishino Stone

P311-4A61C4C    W0133-1768191  New

Qty:
42.43
Ishino Stone Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • Spark plug well seals and cam plugs are not included (order separately).
  • Left
Brand: Ishino Stone
Additional Fitment Information:
Vehicle Aspiration
2004 - Subaru Forester Turbocharged
Ishino Stone
2014 Subaru Impreza Engine Valve Cover Gasket 4 Cyl 2.0L Ishino Stone

P311-4B530B4    W0133-1995489  New

Qty:
37.95
Ishino Stone Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
Brand: Ishino Stone
Additional Fitment Information:
Vehicle Block Engine CID CC
2014 - Subaru Impreza H 4 Cyl 2.0L - 1995
Ishino Stone
2009 Subaru Forester Engine Valve Cover Gasket Ishino Stone

P311-22E1604    W0133-1799028  New

Qty:
37.35
Ishino Stone Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • Cam plugs are not included (order separately)
Brand: Ishino Stone
Additional Fitment Information:
Vehicle Aspiration
2009 - Subaru Forester Naturally Aspirated
Nippon Reinz
1998 Subaru Legacy Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-249C2BC    W0133-1634563  New

Qty:
21.73
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Eng: 993722-, Production: -07/31/1997
  • Subaru used 2 Different Valve covers during June of 1998. Valve Covers are marked inside - For engines from mid 5/98 (from engine #285352) through the end of 1998 production, either gasket #13270 AA082 or #13270 AA111 may be required, depending on the valve cover installed. If the engine is equipped with RH valve cover #13264 AA160, use gasket #13270 AA082. If the engine is equipped with RH valve cover #13264 AA190, use gasket #13270 AA111.
  • Right
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Engine Range Prod. Date Range
1998 - Subaru Legacy H 4 Cyl 2.5L - 2458 993722 and up To:07-31-97
Nippon Reinz
1998 Subaru Legacy Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-249C2BC    W0133-1634563  New

Qty:
21.73
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Eng: 047283-285351, Production: 09/01/1997-05/31/1998
  • Subaru used 2 Different Valve covers during June of 1998. Valve Covers are marked inside - For engines from mid 5/98 (from engine #285352) through the end of 1998 production, either gasket #13270 AA082 or #13270 AA111 may be required, depending on the valve cover installed. If the engine is equipped with RH valve cover #13264 AA160, use gasket #13270 AA082. If the engine is equipped with RH valve cover #13264 AA190, use gasket #13270 AA111.
  • Right
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Engine Range Prod. Date Range
1998 - Subaru Legacy H 4 Cyl 2.5L - 2458 047283 and up 285351 Fr:09-01-97 To:05-31-98
Nippon Reinz
1999 Subaru Impreza Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-249C2BC    W0133-1634563  New

Qty:
21.73
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Production: -05/31/1998
  • Subaru used 2 Different Valve covers during June of 1998. Valve Covers are marked inside - For engines from mid 5/98 (from engine #285352) through the end of 1998 production, either gasket #13270 AA082 or #13270 AA111 may be required, depending on the valve cover installed. If the engine is equipped with RH valve cover #13264 AA160, use gasket #13270 AA082. If the engine is equipped with RH valve cover #13264 AA190, use gasket #13270 AA111.
  • Right
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Prod. Date Range
1999 - Subaru Impreza H 4 Cyl 2.5L - 2458 To:05-31-98
Nippon Reinz
1999 Subaru Impreza Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-249C2BC    W0133-1634563  New

Qty:
21.73
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Production: 05/01/1998-06/30/1998, Type 1
  • Subaru used 2 Different Valve covers during June of 1998. Valve Covers are marked inside - For engines from mid 5/98 (from engine #285352) through the end of 1998 production, either gasket #13270 AA082 or #13270 AA111 may be required, depending on the valve cover installed. If the engine is equipped with RH valve cover #13264 AA160, use gasket #13270 AA082. If the engine is equipped with RH valve cover #13264 AA190, use gasket #13270 AA111.
  • Right
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Prod. Date Range
1999 - Subaru Impreza H 4 Cyl 2.5L - 2458 Fr:05-01-98 To:06-30-98
Nippon Reinz
1997 Subaru Legacy Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-249C2BC    W0133-1634563  New

Qty:
21.73
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Eng: 047283-285351, Production: 09/01/1997-
  • Subaru used 2 Different Valve covers during June of 1998. Valve Covers are marked inside - For engines from mid 5/98 (from engine #285352) through the end of 1998 production, either gasket #13270 AA082 or #13270 AA111 may be required, depending on the valve cover installed. If the engine is equipped with RH valve cover #13264 AA160, use gasket #13270 AA082. If the engine is equipped with RH valve cover #13264 AA190, use gasket #13270 AA111.
  • Right
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Engine Range Prod. Date Range
1997 - Subaru Legacy H 4 Cyl 2.5L - 2458 047283 and up 285351 Fr:09-01-97
Nippon Reinz
1999 Subaru Legacy Engine Valve Cover Gasket 4 Cyl 2.5L Nippon Reinz

P311-249C2BC    W0133-1634563  New

Qty:
21.73
Nippon Reinz Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • ; Eng: 047283-, Production: -05/31/1998
  • Subaru used 2 Different Valve covers during June of 1998. Valve Covers are marked inside - For engines from mid 5/98 (from engine #285352) through the end of 1998 production, either gasket #13270 AA082 or #13270 AA111 may be required, depending on the valve cover installed. If the engine is equipped with RH valve cover #13264 AA160, use gasket #13270 AA082. If the engine is equipped with RH valve cover #13264 AA190, use gasket #13270 AA111.
  • Right
Brand: Nippon Reinz
Additional Fitment Information:
Vehicle Block Engine CID CC Engine Range Prod. Date Range
1999 - Subaru Legacy H 4 Cyl 2.5L - 2458 047283 and up To:05-31-98
Ishino Stone
2007 Subaru Outback Engine Valve Cover Gasket 6 Cyl 3.0L Ishino Stone

P311-1B3D393    W0133-1772397  New

Qty:
28.06
Ishino Stone Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • Spark plug well seals and cam plug ½-moons are not included (order separately).
  • Right
Brand: Ishino Stone
Additional Fitment Information:
Vehicle Block Engine CID CC
2007 - Subaru Outback H 6 Cyl 3.0L - 3000
Ishino Stone
2013 Subaru Impreza Engine Valve Cover Gasket 4 Cyl 2.0L Ishino Stone

P311-45E6BD9    W0133-1917891  New

Qty:
38.42
Ishino Stone Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • Right
Brand: Ishino Stone
Additional Fitment Information:
Vehicle Block Engine CID CC
2013 - Subaru Impreza H 4 Cyl 2.0L - 1995
Ishino Stone
2012 Subaru Tribeca Engine Valve Cover Gasket Ishino Stone

P311-49E9D45    W0133-1797059  New

Qty:
38.21
Ishino Stone Engine Valve Cover Gasket
  • This Original Equipment Manufacturer part is the same part that was made & installed by the car manufacturer at the factory where the car was produced.
  • Spark plug well seals and cam plug ½-moons are not included (order separately).
  • Left
Brand: Ishino Stone
Additional Fitment Information:
Vehicle
2012 - Subaru Tribeca

Latest Car Advice

CarJunky AutoAdvice

Oil leak in head gasket..

Showing 2 out of 9 Posts | Show 7 Hidden Posts
Question From tafkadb on Oil leak in head gasket..

Car is a 2002 Subaru Forester, 110k. Had it in yesterday to determine why there were small amounts of oil forming under it during the last week. Part of the problem was the result of a poor job done during the last oil change and is now fixed, but the dealer found a very minor oil leak in the head gasket.

I know next to nothing about auto repair, and I'm wondering what everyone here would expect to pay to get this fixed. I was given a quote of 1800. I'm also wondering whether this is something that has to be fixed (the mechanic seemed to indicate that it is not urgent but ought to be fixed if I planned on driving the car for another 100k or so) or if I could let it go or even do some non-expert maintenance to it to avoid any problems.

Thanks.

Response From mikethomsun

Oil leakage in head gasket is due to several reasons, when the joint surface is not cleaned propley or damage, The cylinder head bolts or nuts have been tightened incorrectly, another cause of warping.





NO SPAM

Response From Tom Greenleaf

Appreciate your saying you knew next to nothing as that helps us help you!

What went wrong with this last oil change to cause you to saying that it was poorly done? Did they spill oil? Did you complain or bring up an oil leak where you park or something?

Yes a head gasket could leak oil but so far I suggest another option based on what you said.

Without fail - learn how to check your own oil manually, watch it and all gauges and warnings till a credible diagnosis has you convinced,

T

Response From tafkadb

Basically, I was finding spots of oil under my car. So, I took it in to get checked and they found some issues.

I don't know what exactly was wrong, but they changed the oil while it was in and said that once they changed it the issue was gone. So, that is totally resolved and not an issue any longer

The other source was the head gasket. It has very minor leaks on both sides and even this morning I found small spots of oil in my driveway.

I'm mainly wondering what I should reasonably expect to pay to get it resealed or beyond that, if I even need too.

Response From zmame

like Tom said clean it very well I would also add dye to the oil so you can pinpoint where it's coming from. I do know that those Subaru boxer style engine are know for headgaskets, but not sure how they leak internaly or externaly.

Response From Tom Greenleaf

Z - anyone: I think they can leak any dang way they feel like it but both the exact same way at the same time I just don't find very common??

Tom

Response From zmame

I guess I should have worded that better. I don't know which way the headgaskets commonly leak.. as far as weather it's the head gaskets or not if they did not degrease the engine and re-inspect then it's not properly diagnosed(in my opinion). I would get a second opinion for sure odds are it wouldn't be leaking from both head gaskets, but weirder things have happened.

Response From Mr.scotty Top Rated Answer

And they are sure it's not just the valve cover gasket leaking down on to the head making it look like the head gasket is leaking?
I would clean the engine up the best i could then drive it for a few days and keep checking and see if you can see what's leaking before it drips onto anything else.

Response From Tom Greenleaf

I'm not there looking which hurts for something like this. I just find it interesting that two head gaskets both are leaking just this way?!

Anything is possible but head gaskets can leak anything they seal in any direction and for two to have the exact same issue and you would do both if one was bad just seems unlikely to me.

If you can identify the spot, clean it - heck - put talcum power on it and watch the trail or get a diagnosis that's credible as so far I'm not totally convinced and do think techs are mostly not trying to make up work for no real reason and not saying that but would like to see this diagnosed for sure, not guessing,

T

1990 Subi Legacy EJ22 SOHC overheating

Showing 2 out of 2 Posts
Question From subisnacks on 1990 Subi Legacy EJ22 SOHC overheating

Hi, I have a 1990 Subaru Legacy wagon with EJ22 SOHC engine. It has been overheating, mechanic told me it was the water pump. So I had a car project weekend & here's what all I fixed: new water pump, new thermostat, new timing belt & idler pulleys, new valve cover gaskets, new oil filter & oil change, new coolant, new radiator hoses & clamps, new alternator belt. After all that work its still overheating, it starts up & temp starts to go up, but it doesn't stop at operating temp it just continues to go up & up & in about 15 minutes the temp is thru the roof (either at idle or while driving, driving seems to get it there faster). So I'm thinking clogged radiator? Any other ideas?

Response From nickwarner Top Rated Answer

You could have a plugged radiator, could've put the thermostat in backwards, could have blown a head gasket or may be as simple as you haven't gotten the air bed out of the system yet. Double check your work.

1997 subaru legacy GT

Showing 2 out of 2 Posts
Question From kkaiser80 on 1997 subaru legacy GT

So I have a question. My car is throwing codes for both knock detonation sensor and o2 sensor are bad. I haven't noticed a loss in power but the car has probably had this ussue since I got it. So it could have a loss in power. Only thing that I am noticing is lately a rough or not stable idle, I start my car once and it starts perfect. I try again and it stutters,, turn it off try again and it starts up fine, turn it off and try it again it sorta stutters in RPM's. Its really inconsistent. I haven't had the car long enough to notice if im getting bad gas milage because in comparison to my last car it is getting awesome mileage. I do notice a strong odor from exhaust almost like fuel. Meaning it is getting to much fuel which I have heard is related to the bad knock sensor. I just want to know which one I should try replacing first that corresponds with my symptoms. My car start everytime, its just iffy on the idle when doing so. And it gets better as the car gets warmer. Its also throwing codes for misfires in cylinder 3/4 every other time I run the obd2, so maybe a spark plug change aswell? Somone said pil might be leaking into one of the spark plugs, and I have noticed a smell of oil burning somewhere in the engine bay, but very minimal because I don't lose very much oil at all. Barley have to add any between 3000 mile oil changes. 1997 Subaru legacy 2.5 gt. TIA.

Response From nickwarner Top Rated Answer

First, realize that codes are not a magic ball and do not tell you that a sensor or other part is bad. There are upwards of 20 different codes related to an O2 sensor but most do not even indicate a bad sensor. The codes point to a circuit and give a direction to look in. They are a start to a diagnosis, not a diagnosis in itself.
On an OHC engine the plugs are down in some wells in the valve cover. The gasket has seals that go around each of those tubes. If they leak then oil gets down around the plug and shorts it to ground through the block instead of the full power going through the spark plug. You should look down the wells and see if this is happening. If it is, a new valve cover gasket will be needed, and you should change the plugs and wires while you are in there. If there is a lot of oil in the plug tube, you should suction as much out as you can before you pull the plug out so you don't dump a bunch into the cylinder. If a lot of oil is in there, you could potentially hydrolock it when you go to start it later. At minimum it will smoke like hell for a few minutes.
You are smelling fuel out of the exhaust because you have a misfire and that gas didn't get burned in the cylinder. It is important to fix that before you damage your catalyst from the raw fuel entering it or wash down a cylinder with fuel and wipe the rings out. Also, some of the fuel will wash down into the oil so when you are getting rid of the misfire you should change the oil. Oil watered down with gasoline doesn't lubricate well and will wreck the rest of the motor.
Check out the plug wells and condition of the spark plugs, and also please post back with the exact code numbers you are getting so we have a direction to go with the diagnosis.

p0420 2006 Nissan Altima 2.5

Showing 4 out of 4 Posts
Question From soobie on p0420 2006 Nissan Altima 2.5

I'd appreciate some insight on determining what to do with a friend of mine's 2006 Nissan Altima with the 2.5 liter 4 cylinder engine. His car currently has around 100,000 miles and I know this person is very lax with car maintenance. He hasn't performed a tune up, hasn't replaced any of the 02 sensors yet, the valve cover gasket is leaking a bit, timing chain is still original too along with all of the exhaust components. The car has thrown a p0420 code. From what I've been able to discern, Nissan's variable valve tech at times allows both the intake and exhaust valves to be slightly open at the same time to a certain degree and if the precat material has been damaged due to the engine running richly, backpressure causes particulates will get into the engine from the exhaust manifold and wear the insides out- possibly resulting in the need for major repairs or a new engine. On a side note, I'm curious if other makes are vulnerable to the precat material potentially causing damage to the engine- especially on Subarus. I currently own a 2007 Impreza 2.5i. I'm curious if certain makes are not know to be vulnerable to this, why?

If pressure check ok, I was thinking of replacing the precat with a catless header with locations on it for the 02 sensors. I however don't know how this could affect the engine- run lean or rich and it could damage the other catalytic converter too. I'd also prefer that the warning light goes out and doesn't stay on due to the mod if can be helped this way. Recommendations on this? New stock? There aren't any other codes that would indicate 02 sensors needing replacement specifically either. Is there a convincing way to prove to somebody if they are going bad or not? From what I've read around the 02 sensors get lazy over time and don't respond quickly possibly resulting in an engine the runs too richly at times.

Also, I'd like to perform a compression test on the vehicle, but have never done so. How many times do I need to crank the ignition in order to get reliable readings on the pressure gauge? I could use some clarification on this part.
He actually thinks that he'll be alright to not replace anything and he may still be able to drive it another 70,000 miles or so which I have a hard time agreeing with.
I appreciate your thoughts

Response From Hammer Time

What you are talking about doing is against federal law and likely won't do a thing for your problem. I don't know why you're assuming it's a problem with a pre-cat. The PO420 is telling you that the cat is not doing it's job any more. You just need to replace the cats. It's not the design of the engine that did this. It's being so lax on the maintenance that causes the failure.

Response From soobie Top Rated Answer

So if the precat is bad, a different code from p0420 would pop? I know it may be one of many possibilities, but I explained why I think this is a good idea because of the potential consequences for his engine, this circumstance is real and has happened to many others-
and me considering that he may not be on top of preventative mx items. How do you verify the o2 sensors are working up to spec still? If not, can I rely on a related code to pop up right away or does it have to reach a certain extent.

I've been told that he could still pass emissions with a gutted precat or aftermarket header on an Altima forum. I don't know for sure, but that's certainly something to consider- thanks for mentioning it.

I know what the code means and just replacing the cats does not guaranty a solution. If there's an underlying cause, the cat/cats could become unserviceable again.

What other things should I be looking at?

And how can I check and justify expenses for misc part/s through empirical data if possible?

Response From Hammer Time

First off, the pre-cats are not monitored independently so you have no way to know if it's the pre-cat with the problem or not,..... likely not. The Cat monitor is located after the main Cat.

Cats die from contamination like raw fuel, burned oil and coolant that passes through them. If you want your cat to last longer, just keep the engine running at optimum.