p0420 within 600 miles every time

[sirtokesalot]

 so my car has been throwing p0420 codes around every 600 miles give or take a few is the p0420 code a definite bad catalytic converter? ive been putting off doing anything about it untill this summer when i pull the engine for head gaskets and any other gaskets it might need. i have the cats from my old car that the engine came from they were not throwing any codes in the old car would just putting the other cats in stop the code?

[matt167]

Vac leaks or a poor running engine could also cause it.

[Fairtax4me]

I remember you had a thread about the swap a while back but did you do anything like plugs, wires, filters, PCV valve?

 

Bad O2 sensors can cause that as well. Any exhaust leak between the heads and the cats, and occasionally have seen this caused by leaks at the flange behind the rear cat.

 

Lots of things to check before replacing cats, but should the cats turn out to be bad, the set from your other car should swap in just fine.

[sirtokesalot]

I remember you had a thread about the swap a while back but did you do anything like plugs, wires, filters, PCV valve?

 

Bad O2 sensors can cause that as well. Any exhaust leak between the heads and the cats, and occasionally have seen this caused by leaks at the flange behind the rear cat.

 

Lots of things to check before replacing cats, but should the cats turn out to be bad, the set from your other car should swap in just fine.

before the swap took place i had done plugs and wires i did not do pcv valve there is a small exhaust leak at the flex pipe after the cats and o2 sensors i figured it would not cause the code though being behind the cats i might just change them anyways when i do the engine because ill have to cut and weld a new flex anyways how much harder is it to unbolt the cats and bolt the other cars one in before welding the flex in. was just curious on the code i want to be sure it goes away. the engine did not have any vacuum issues before the swap and the 420 code is the only code coming up for the last 5 check engine lights before that it was alternating between p0440 and p0420 i haven't seen p0440 in over 3000 miles now.

[DaveT]

Our 01 Forester had the intermittent 0420 code.  New O2 sensors had no effect.  Turned out to be a small exhaust leak in the cat pipe.  Any leak between the 2 O2 sensors would cause the code.

 

Yes, it can be caused by other things as well.  Failing cats is one for sure, as that is what the 2nd O2 sensor is there to do - verify operation of the cat.

[wirelessenabled]

I think the P0420 code is due to bad programming on Subaru's part.  On my 2000 OBW, purchased new in March, 2000, I have had the P0420 codes intermittently since about 60K miles in probably 2005 or 2006.  Now at 230K miles I can get the code once a week or so for a few weeks, then I won't see it again for months.  I find it hard to believe that a small leak would come and go in this fashion.

 

I think Subaru programmed the numbers wrong and a very small change in temps, humidity, fuel, cause this parameter to go out of bounds and set the code.

 

I have replaced O2 sensors with no change in the pattern.

 

If my cats were bad they would be long since been completely non-functional with the result that the code would be set all the time.

 

But no!  The code comes and goes randomly as it does on my son's 2000.

 

I carry a code reader in the car and can reset the code when it occurs so I know how often it is set. 

 

If the code were continuous I would be working on it but the code is not continuous but rather it is random and I believe in error.

[Dj7291993]

Make sure they're are no running issues with the engine, or leaks in the exhaust.  Sometimes, running something like BG 44K through can help if it's just carboned up, but it's not 100%.  O2 sensors can cause it, though in many cases, it ends up being the cat.  Just keep in mind that, many times, it wasn't the cat's fault.  If you have a way to watch the O2 sensors, either through a fast enough scanner or a scope, you can see what the computer sees.  For the most part, except in wide open throttle and closed throttle decel, once the cat is warmed up, the back sensor should be a flat-line.  Exactly how much movement is allowed varies by manufacturer, but a good quality new cat will have none.  The variance in setting the code can come from how much the cat has to deal with.  For example, the cat's capacity doesn't need to be nearly as big to handle idle as it does cruising at 45mph.

 

Also, my experience is from Colorado, where rust isn't as much of an issue, so we don't see as many of them caused by leaks.

[DaveT]

In the case I experienced, it wasnt that the leak came and went, it was a small leak, that shifted the reading to be right on the edge of the code set limit, then any small variable could trip it on and off.

[sirtokesalot]

I have repaired the exhaust leak p0420 came back this morning driving to work 386 miles since the last time. I suppose I probly should have swapped the o2 sensors while I was under the car fixing the leak.

Edited March 25, 2017 by sirtokesalot

[Dj7291993]

I have repaired the exhaust leak p0420 came back this morning driving to work 386 miles since the last time. I suppose I probly should have swapped the o2 sensors while I was under the car fixing the leak.

 

Depending on the year, that may not be an option.  I know my 2000 Forester uses an Air Fuel Ratio sensor in the front, which won't interchange with a normal o2 sensor.  The connectors are different though, so you would notice if you tried.

 

Edit: Right as I hit post, I realized you probably meant replace.  Have you tried running a cleaner though the fuel like 44k?

Edited March 25, 2017 by Dj7291993

[Caboobaroo]

Like I tell people at work. Start by running a good quality top tier gas, either plus or premium. Next step is to check O2 readings and replace the front sensor IF everything else is square. IE no vacuum leaks, spark plugs are good quality and within gap spec and no exhaust leaks.

If that still doesn't fix it, then probably time for a new catalytic converter. You can do the anti fouler trick but it's more of a bandaid then anything.

[CNY_Dave]

These computers are set up right on the edge, the cat can be good, no vacuum leaks, AF mixture in spec, and it'll still trip an 0420.

 

After an actual problem is ruled out, nothing wrong with the anti-fouler method, since there's no actual problem.

[idosubaru]

I think the P0420 code is due to bad programming on Subaru's part. .

Nope, they're universal - other manufacturers are not exempt from P0420 code debacles.  

 

Whatever entities set forth these emissions requirements may be to blame, but I doubt it's entirely Subaru desiring this (if at all).

 

 

After an actual problem is ruled out, nothing wrong with the anti-fouler method, since there's no actual problem.

 

Yep - in MD they'll pass the actual sniffer emissions test.  so they're passing legal emissions requirements but requiring more converter replacement which increases energy demands - requiring more mining (energy), recycling demands (energy), smelting (energy), welding (energy), processing (energy), logistics (energy), shipping (energy), installation (energy), lower vehicle values and higher turn over (energy)....

 

i feel *really* bad circumventing P0420 codes.  LOL

[lmdew]

Fixed it on my Daughters 99 Impreza OBS.  the short extension on the read O2 Sensor did the trick.

[sirtokesalot]

the whole thing is that this engine came from a car that i had driven for 3 years without ever seeing the check engine light on for anything except a speed sensor. this makes me think something on the car is bad. the engine had new plugs and wires before the swap occurred and from what i could tell there's not much vacuum related to leak all the hoses i used were in good shape. the only differences are that i used the cats and o2's that were on the 98 instead of the ones that were on the 95 although i did save the 95 ones and i used the 98 air box instead of the 95 air box witch i also have.

[wirelessenabled]

To add some more info, when I replaced the transmission in my son's 2000 Outback last year I checked out the exhaust system very carefully since it was removed.  There was no sign of any leaks.  I reassembled the system with all new gaskets.  Result was a continuation of P0420 codes on the same random schedule.

[GeneralDisorder]

Just get one of these:

 

https://www.amazon.com/gp/aw/d/B00B1U6I54/ref=pd_aw_fbt_263_img_2?ie=UTF8&psc=1&refRID=7EE0Y5Z5X0CZ2WGTZ2HG

 

Or a new dealer cat....

 

GD

[sirtokesalot]

quick update i replaced a flex joint in the exhaust that was leaking and have been driving around i have about 4500 miles  since the last time it came on.

[idosubaru]

quick update i replaced a flex joint in the exhaust that was leaking and have been driving around i have about 4500 miles  since the last time it came on.

 

nice hit.  

 

what do you mean by "flex joint" - in front of the converter or....?

 

poor exhaust is a definite issue and us in the rust belt are prone to lots of exhaust issues. 

[sirtokesalot]

it was behind the converter and at the time it was not leaking badly but as it got louder it pushed me to fix it. i know it was not supposed to be a flex joint there but thats what was on it and it was leaking so i just replaced it with another flex joint.

[golucky66]

A leak after the rear O2 set a P0420?

Didn't think it mattered at all So long as the leak is after the rear O2

[ferret]

Just replace the Cats.... Here's a company I have used from Ebay for 3 replacements thus far. Not one has come back with problems. They Fit EXACTLY as original, unlike many after market exhausts where they are 1/2' too short, or the bend is NOT QUITE CORRECT.

 

First one now was over 2 years ago with no problems. They come out of Toronto and have a 5 year warranty. Below is a link for the last one I used for 2000-2005 Outback. But visit their store for others:

 

http://www.ebay.com/itm/2000-2005-Subaru-Outback-2-5L-Direct-Fit-Catalytic-Converter-Gaskets-/263046061872?vxp=mtr&hash=item3d3ec45f30

[Fairtax4me]

A leak after the rear O2 set a P0420?

Didn't think it mattered at all So long as the leak is after the rear O2

A leak at the joint directly behind the rear cat can pull enough air in to alter the reading of the rear sensor.

[golucky66]

So what's you're saying is a leak AFTER the cat and BEFORE the rear O2? Because that makes sense.

I'm wondering about after the rear O2. Can that actually alter the rear O2's readings?

[Fairtax4me]

So what's you're saying is a leak AFTER the cat and BEFORE the rear O2? Because that makes sense.

I'm wondering about after the rear O2. Can that actually alter the rear O2's readings?

It can be after the rear cat and the rear o2 sensor. When the engine is decelerating (slowing to a stop or going downhill, foot off the pedal) air will travel up the exhaust stream towards the engine. Even though it's only for a short time, it's sometimes enough to change the reading of the rear sensor enough to cause the code to set.