hey Nipper: 420 code

[timintc]

Im lucky here in Northern Michigan to have an outfit called The Subaru Guys who do nothing but Soobys and at a far more reasonable rate than the local dealer and who is kind enough to always answer any questions that i have concerning my 2000 legacy or others that come up from time to time on this forum. I have been dealing with a cel and 420 code for probably a year and a half now, it comes and goes, it was on for a week or so then on a trip to Detroit over the weekend it reset itself on its own and now is off again. On his recommendation i have thus far chosen to do nothing insofar as replacing any o2 sensors, his opinion is that after seeing a number of these situations hes not so sure which sensor front or back is the culprit, but his current theory is now that it is the back sensor, and when he replaces them he welds in a small spacer where the sensor goes in to back it away from whatever it is that it is monitoring, his reasoning being that Subaru set the sensitivity in a manner that any little glitch in the system would trigger a cel and thus a trip to the dealer. I am tempted to test his theory and let him have a go at it cuz the cel is annoying specially with the kids always having to tell me " hey dad your check engine light is on". i have had auto zone type guys tell me after reading the code that i have to have my cat replaced, i pity the poor uneducated folks who would follow that advice, my car runs the same weather the light is on or now and with the price of oem o2 sensors why replace something that doesnt need be.

[northguy]

420 code? Is this a secret message that the "stash" has arrived?

[Milemaker13]

The car wants to get high when ever that code comes up. I suggest pulling to the side and getting chinese eyes!

[nipper]

ok that caught my attention, put it in the special oregeno box....

 

If your car has over 80,000 miles on it, suspect the front o2 sensor.

The front one communiates with the fuel injection. The rear one monitors the outpu of the cat by comparing it with the front one When the front one goes wacky, the rear not knowing better think its the cats fault.

 

If you do a search here you will see this is typical and replacing the front one fixes the problem.

 

 

nipper

[Tech1967]

9 out of 10 times code 420 means something different here in British Columbia but since you seem to be talking about your car...

I check both o2s waveforms and the gas analyser when it was available to see if one was trying to tell a lie. I'd check the flow at the tailpipe to see if it was low at 2500-3000 rpm, gotta be bad for that though and you'd probably hear the difference. Final step if I had a temp gun would be to look for a substantially higher temp at the rear of the cat compared to the front. Sadly as a dealer shop I'm still missing two of these tools so I understand the difficulty of duplicating this in the driveway. Good luck.

[WAWalker]

Hate to say but the Auto Zone guy was correct. To FIX the problem the cats will need to be replaced.

 

Your guys method of putting a spacer between the rear O2 sensor and cat will keep the CEL out (the sensor itself is probably fine) but is only masking the problem. It is not FIXING the problem. But if all you want is the CEL to stay off then it will work. NOT NESSASARALY LEGAL, but will work.

 

The rear O2 sensor is there to monitor the catalist efficency.

When not in the exhaust stream it can not read the actual O2 levels of the after cat exhaust so the computer will think everything is fine.

 

Side note: The CEL was not put in cars buy the manufature to get you to go to their dealer every time it comes on. The fedral government told them they had to put it there. OBD is there primairaly to monitor emissions.

[Skip]

WA - okay lets throw 700 + $ at the problem and hope it goes away?

If a 02 sensor is the culprit who is the looser? Not you!!

 

I like following -- test the outputs - then replace --

type advise.

 

Do as you like, but these type statements get under my feathers.

 

One should not state what they think is true -- when it may not be fact.

 

Sorry for being abusive.

[nipper]

Now what i do for people is replace the front o2 sensor if it has never been replaced before. The reasoning is even if it is the cat, and its the original o2 sensor, your still going to need a new sensor anyway.

It's rare for a cat to go bad on these cars (but it does happen).

 

nipper

[Manarius]

I think if the cat was bad, you'd notice. The rotten egg smell would be...overwhelming.

[OB99W]

Im lucky here in Northern Michigan to have an outfit called The Subaru Guys who do nothing but Soobys and at a far more reasonable rate than the local dealer and who is kind enough to always answer any questions that i have concerning my 2000 legacy or others that come up from time to time on this forum. I have been dealing with a cel and 420 code for probably a year and a half now, it comes and goes, it was on for a week or so then on a trip to Detroit over the weekend it reset itself on its own and now is off again. On his recommendation i have thus far chosen to do nothing insofar as replacing any o2 sensors, his opinion is that after seeing a number of these situations hes not so sure which sensor front or back is the culprit, but his current theory is now that it is the back sensor[...]

It's nice to have a competent independent mechanic. However, sometimes using the dealer makes sense. The 2000 had problems with O2 sensors, and perhaps you can have yours replaced at no charge. As has been already stated, a sufficiently problematic front sensor can cause the P0420 trouble code. Take a look at the link below, in particular the "WXW80" entry. Call SOA and/or a dealer, mention WXW80, and see what they have to say.

 

http://www.carcomplaints.com/Subaru/Legacy/2000/tsbs.shtml

[Tech1967]

I think if the cat was bad, you'd notice. The rotten egg smell would be...overwhelming.

 

That used to be true but obd 2 requires 90% catalyst efficiency I think. Now they can fail without rattleing, stinking, or reducing performance. Nipper makes a certain amount of sense though because there are certain upstream requirements for proper catalyst functioning. You can run into a garbage in garbage out situation which is why I make sure everything is going right in front before condemning them. A quick fix to make the light go out may be fine in some places but during the years I spent doing Aircare repairs it would have made a lot of people mad. No pass, no drive.

[WAWalker]

WA - okay lets throw 700 + $ at the problem and hope it goes away?

If a 02 sensor is the culprit who is the looser? Not you!!

 

I like following -- test the outputs - then replace --

type advise.

 

Do as you like, but these type statements get under my feathers.

 

One should not state what they think is true -- when it may not be fact.

 

Sorry for being abusive.

 

PATTERN FAILURES are a very real thing.

 

Bad converters have been found to be the biggest cause of the P0420 code on these cars by many PROFESSIONAL automotive tech for all over the country.

 

I would suggest testing the O2 sensors before replacing the cats, but DIYers and even a lot of "professionals" don't have the ability to check the front sensor. It is an Air Fuel Ratio sensor and can not be check the same way as an O2 sensor. So a lot of A/F sensor get replaced with out knowing if they are good or bad, it is just the cheapest first step. Personally I do test the sensors, since I have the Select Monitor I am able to do that. Yes there was a recall on a lot of the A/F sensors, not because they were causeing a P0420 code to set.

 

Most of what is on this board is people stating WHAT THEY THINK, very little experiance to back any of it up. Your feathers should be plenty ruffled.

[nipper]

Odd not from my expierince are 420's caused by catalytic failure, especially in the USA , and not from what ive seen on this board. Do a search youll see that 98% of the p0420 codes are a bad front o2 sensor.

Catalytic converter failure in the 1980's was very common, but since the 1990's and on up its not all that common, unless there is a design fault or a bad tank of gas. Since what geos in the cat is fairly standarized with all manufacturers, and what comes out is standardized, they have become trouble free. A bad front o2 sensor (if it is really out of wack) can burn up a cat. Not everyone has the tools to fully diagnose a cat issue, but one can safely assume that if a specific pattern happens its the front o2 sensor. If the problem starts off as an occassionally flashing CEL, then at some time the light stays on, thats is usually an o2 sensor, then add in the cars mileage, and the trouble code (only one 420) the o2 replacement usually rectifies the situation. A bad cat on the other hand will just fail, and not ocassionally throw a CEL light. Also a bad cat when you hit it with your hand will rattle, since it means the catalyst has solidified and broken apart. the other way a cat can fail will cause a performance issue.

Also the trouble codes are a diagnostic tool, they have never meant to be the end all in diagnostics.

Maybe im just lucky, being a dying breed that learned car repair as emission controls came on line, so I got to see the transition from pcv/egr valave, through the ugly 1970's the goofy 1980's to OBD1 and OBD2. For anyone who is counting year, my father was a mechanic, and i was always fascinated with cars.

 

 

What is wrong with thinking anyway?

 

Sorry if i havent pulled a wrench in a few years (about 5 though still do it on occassion), but ive been pulling one since i was 6, and am now 45. I'm an automotive engineering proffesional (been a tech, and am a degreed engineer) I Have worked with the federal EPA test aqupiment. I am still a licensed state inspector in the state of NY. I am a member of SAE and API and ASE for about 15 years, well i can go on.

Just because some of us dont list our qualifications on every post, does not mean that we don't know what we are talking about.

Also i keep up on all the information out in the real world, as well as trade information.

 

I like it when people think out loud, as it explains thier reasoning, and then you know how to TEACH them where their reasoning is flawed.

And besides I just like to teach.

 

nipper

[ferret]

In another thread, I posted my own experiences. Take it for what you want, this is what I have done and experienced:

 

http://www.ultimatesubaru.org/forum/showthread.php?t=68027

 

My own experiences with this code across many different vehicles ( not only Subarus ):

 

1. Look for exhaust leaks anywhere from the engine to the rear oxygen sensor. Many times I have found this to be the biggest cause.

 

2. Failed or failing Catalytic converter. Owners usually don't want to hear this, but unless there is a leak, or slow responding oxygen sensor ( front or rear ), the code is correct when it is set. If you reset this code without fixing anything, it will usually come back in a day...2.....week....next 2 weeks....The converter should be replaced.

 

3. Cheaper than the converter but usually not the cause, replacing the front, rear or both oxygen sensors. Without a code pointing directly at the sensor, or charting the sensor to prove it has a slow response, this is cheaper than replacing the converter, but in most of my experiences, it just puts off replacing the converter.

 

I have seen converters last years and 150K + miles. But it seems the ones that were part of the 99 and newer vehicles are of a different substrate and don't seem to last as long. Somewhere in this time, car manufacturers had to lower emissions again and introduced a slightly different converter and oxygen sensors. Both don't appear to be a long lasting as in the past.

 

My own experiences....and $.02. Take it for what it's worth. You are welcome to input with your own.

[WAWalker]

 

 

If the problem starts off as an occassionally flashing CEL, then at some time the light stays on, thats is usually an o2 sensor,

 

A flashing CEL indicates a Class A Misfire. Show me one proven case were a bad O2 sensor caused a Class A Misfire.

 

A bad cat on the other hand will just fail, and not ocassionally throw a CEL light. Also a bad cat when you hit it with your hand will rattle, since it means the catalyst has solidified and broken apart. the other way a cat can fail will cause a performance issue.

 

In your mind. Has been proved differently in many real work situations.

 

Also the trouble codes are a diagnostic tool, they have never meant to be the end all in diagnostics.

 

Truer words have never been spoken.

 

Maybe im just lucky, being a dying breed that learned car repair as emission controls came on line, so I got to see the transition from pcv/egr valave, through the ugly 1970's the goofy 1980's to OBD1 and OBD2. For anyone who is counting year, my father was a mechanic, and i was always fascinated with cars.

 

Yes, many of the old timers are getting out of the business. Getting harder to keep up and be able to efficiently diagnose and repair cars.

 

Sorry if i havent pulled a wrench in a few years (about 5 though still do it on occassion), but ive been pulling one since i was 6, and am now 45. I'm an automotive engineering proffesional (been a tech, and am a degreed engineer) I Have worked with the federal EPA test aqupiment. I am still a licensed state inspector in the state of NY. I am a member of SAE and API and ASE for about 15 years, well i can go on.

Just because some of us dont list our qualifications on every post, does not mean that we don't know what we are talking about.

Also i keep up on all the information out in the real world, as well as trade information.

 

I knew that you are a engineer. I also know that nobody is perfect, myself for sure. Nothing aginst you, but I also know for a fact that even the people writing TSB's for SOA aren't always right the first time, and I would assume they have some back ground in engineering also.

 

I like it when people think out loud, as it explains thier reasoning, and then you know how to TEACH them where their reasoning is flawed.

And besides I just like to teach.

 

nipper

 

I like to learn and will be learning till the day I die. Makes me better at what I do.

 

 

 

With your resume you should be able to become a member of IATN. $20 a month and you can search the fix data base. I think you would find it a very good resource.

[OB99W]

In a perfect world, OBD-II trouble codes would be totally specific and accurate (seriously reducing the need for trained techs ); the ECU would always correctly determine whether the O2 sensors were operating properly, and then the code would indicate a sensor or catalyst efficiency problem that you could trust without doubt. Unfortunately, this world is far from perfect, and the trouble codes sometimes have to be interpreted.

 

The ECU determines catalyst efficiency by looking at the downstream/rear O2 sensor output; it should be low and steady. When it's not, the ECU sets the P0420 code. Of course, a bad cat alone can cause the problem, but so can other things that sufficiently impact what is "seen" by the rear sensor.

 

A P0420 code could be generated by a combination of marginal catalyst and sensor operation, and in some cases replacing either might cause the code to be cleared (for at least a while).

 

There's little question that a good technician in a decently-equipped shop could likely determine which component is causing the code. The question becomes whether to gamble the cost of diagnosis, believe what P0420 signifies "by the book", or to try replacing a sensor.

 

Reading material; decide the validity of any part of it for yourself:

http://www.tegger.com/hondafaq/p0420.html

[nipper]

A flashing CEL indicates a Class A Misfire. Show me one proven case were a bad O2 sensor caused a Class A Misfire.

 

 

 

I should have been more clear, i didnt mean flashing, i meant intermitent, one that comes and goes. Flashing means missfire or worse, sorry about that

 

IATN, can you please explain the acrnym for everyone else. Right now i'm still fighting insurance companies and SSIDI from the car accident.

(anyone in NY dont get Travelers).

 

nipper

[WAWalker]

International Automotive Technicians Network.

[artistic_gore]

Can the front O2 sensor be replaced with an after market unit? I am throwing this code currently along with a few others that I am trying to remedy. Also if it comes right down to it and I have to replace the cat would this be an acceptable replacement? It says bolt in but for $140 I don't want to be wrong.

[OB99W]

The front O2 sensor is sufficiently critical that we usually suggest it only be replaced with an OEM unit. However, you can usually get an original factory one from an online dealer for around $100, including shipping, so why buy anything questionable for $140?

[nipper]

Have some faith in us.

98% of the 420 codes on this board have been a bad o2 sensor. The o2 sensor (if original) should be replaced with the cat anyway (as long as you are there). You wont be waisting money by replacing the sensor, and yes the front one should be an OE.

 

Secondly, that is way too cheap a cat. It also looks like a cat for a turbo (i am not sure).

 

I wouldnt buy one off ebay.

 

nipper

[cookie]

If I was still a working mechanic I'd probably err on the side of overkill because you don't want customers coming back mad. In my case I got PO 420 and just replaced the front ox sensor, I also purchased the rear but it has been a few years and maybe 30,000 miles and the code has not come back. If it was a customer car I might have done the cats too, but here in CA they are special and quite expensive for my 99.

You can use an atermarket ox sensor but it is often more trouble and in some cases they are not quite right. If a friend had used a cetain model with good results it would make me more likely to try one.

I have used a rebuilt cat in the past with good results and it was far cheaper. I supect as long as it fits and is about the right size it would work.