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#2435399 - 11/15/11 04:09 PM P0420 Code... CAT or O2 sensors?
MysticGold04 Offline


Registered: 01/12/05
Posts: 1133
Loc: CA
2004 Toyota Highlander, 2.4L engine
124,xxx miles.. passed smog earlier this year

My wife was driving when the check engine light came on. I figured it was the gas cap, because I usually fill the tank for her, but she did this time. I had also just changed the air filter, so thought it might be a loose hose too. She kept driving with no issues to Reno, NV for the past weekend. I drove it home yesterday, no issues at all, same good gas mileage, etc. Checked the code, P0420 and cleared.. it will most likely come back.

any good recommendation for an aftermarket Cat? I have looked and the O2 sensors seem to be about $120 to $150 each.

How do I tell whether its the O2 sensors or the CAT?
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#2435425 - 11/15/11 04:31 PM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
Trav Offline


Registered: 11/20/06
Posts: 10044
Loc: MA, Mittelfranken.de
Eastern makes very high quality cats.
Using an infrared thermometer check the temp before and after the cat look for at least 30f, some use to say 100f but some newer engines run so clean there is not that much difference.

Check the coolant temp sensor, there maybe 2 one for the light/gauge and one for the ECM, check the one for the ECM, they usually throw a code if failed but one reading low may not.
Also look for any exhaust leaks before the cat, manifold, pipe etc. Any ingestion of fresh air pre O2 with throw it for a loop.
You really need a scan tool to diagnose this properly, being able to see O2 wave forms and fuel trim is a must, it can also see the other sensors and get a complete picture of what's going on.
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#2435447 - 11/15/11 04:58 PM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
hypervish Offline


Registered: 04/20/10
Posts: 1521
Loc: new jersey
Usually it's the 02 Sensor, rarely do the CAT's go bad.


Edited by hypervish (11/15/11 04:58 PM)
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#2435452 - 11/15/11 05:04 PM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
simple_gifts Offline


Registered: 07/26/04
Posts: 9059
Loc: Middlesex County CT
My !echo! factory manual indicates you can test the O2 sensors with a multimeter. I did not bother and just replaced both of them. (direct fit denso from rockauto) I can't find my factory manual, but will look for it; the procedure is well documented.

I believe the flowchart procedure was

1) verify no exhaust leaks.
2) Check resistance on both O2 sensors (don't know what they "should" read until FM is found)
3) replace cat.

obviously stopping if one of these steps fixes the issue; I disconnected the battery after I performed step #2.


Edited by simple_gifts (11/15/11 05:05 PM)
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#2435469 - 11/15/11 05:26 PM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
simple_gifts Offline


Registered: 07/26/04
Posts: 9059
Loc: Middlesex County CT
http://www.bobistheoilguy.com/forums/ubb...rue#Post2261600

Here is my thread; look @ comments by Bill in Utah.


Edited by simple_gifts (11/15/11 05:27 PM)
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#2435613 - 11/15/11 07:01 PM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
MarkM66 Offline


Registered: 10/19/10
Posts: 1203
Loc: stl
If you really want to be cheap, the old spark plug anti fouler 02 spacer would probably keep that code from coming back.

Not sure if it'd get by your smog check out there though.

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#2435628 - 11/15/11 07:11 PM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
Quest Offline


Registered: 12/19/04
Posts: 6427
Loc: beaver land EH?
IMHO replace the O2 sensors first (pre-cat ones, wideband with pre-heater built in). Get the Denso ones from Rock Auto and if that resolves the problem, then you are all set.

I concur that cat rarely has to be tackled at this stage, mainly O2 sensors.

I don't buy those universal ones that require you to splice it to make it fit. Many a times: it would get you in a very difficult position (like engine would behave oddly even after replacing the O2 sensors with non-OE aftermarket ones).

Q.
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#2436103 - 11/16/11 08:39 AM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
Vikas Offline


Registered: 07/22/05
Posts: 8265
Loc: NorthEast
I strongly disagree with the previous responses. At 124K, it is your CAT which has become *SLIGHTLY* weak. Unfortunately the manufacturer has coded the computer to be very strict and thus you get the P0420 code. This is all a guess unless you do proper diagnosis using good scanner or better and oscilloscope.

If you want to change parts without doing that, then putting another OEM converter will fix it for next 100K miles. If you put an aftermarket, it will remain fix for next 5-10K miles before the light will come on.

There is a lot and I mean *LOT* of information available if you are willing to do the research on P0420.

- Vikas

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#2436225 - 11/16/11 11:12 AM Re: P0420 Code... CAT or O2 sensors? [Re: Vikas]
Trav Offline


Registered: 11/20/06
Posts: 10044
Loc: MA, Mittelfranken.de
I was with you until this..
Quote:
If you put an aftermarket, it will remain fix for next 5-10K miles before the light will come on.


This is simply painting all aftermarket cats with the same broad brush. Many aftermarket cats are junk that's true enough but not all.
Eastern make many OEM cats and sells the same quality even CARB approved and problem vehicle cats.
I put one in a Solara 65K ago and no problem with performance or corrosion.
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#2437173 - 11/17/11 11:07 AM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
MysticGold04 Offline


Registered: 01/12/05
Posts: 1133
Loc: CA
Ok.. here is my thinking, please correct me if I seem wrong in my logic...

Good gas mileage as always - Not the upstream O2 sensor
No hesitation/engine runs strong - Not the Cat

Downstream O2 sensor is failing or failed. Does this seem correct? Aside from checking for exhaust leaks, etc. it seems like the next logical step. At least to me.. a new Denso sensor from Rockauto is $118.
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'04 Highlander 2.4 157,521 Maxlife 5w30
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#2437187 - 11/17/11 11:21 AM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
Quest Offline


Registered: 12/19/04
Posts: 6427
Loc: beaver land EH?
@ MysticGold:

CAT is only responsible for post-combustion exhaust scrub before the gas is vented.

So long as your engine is in good shape (ignition timing, etc.) and the cat matrix hasn't collapsed, there shall never be any perceivable drivability issues detectable.

I'd recommend you to do the following first:

(a) get FSM or similar to find the proper means to test/verify that the downstream (post-CAT) O2 sensor is still within spec?

Once that is done, go get an OBD-II scanner capable of freezeframe (ScanGauge works wonders also) and to monitor and capture some of the values over fuel trim, etc. (snippet obtained from one of the ToyotaTech postings):

P0420 - Catalytic Efficiency under threshold.
RPM: 1944
38.0% Load Value
Coolant 188 degrees
87 degree intake temperature
3.1% Short Term Fuel Trim 1
14.0% Long Term Fuel Trim 1
-100.6 Short Term Fuel Trim 3
-100.6 long Term Fuel Trim 3
49 miles per hour
Fuel System 1: Closed



and post them here to discuss.

Don't discount the possibility of a "hiccup" on the OBD-II monitoring part and simply by clearing the P-0420 code would have resolve this matter entirely w/o even calling on parts replacement...

Q.

a casual reference: http://www.yotatech.com/f2/p0420-code-details-does-mean-anything-anyone-tech-145496/
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#2437227 - 11/17/11 12:05 PM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
Trav Offline


Registered: 11/20/06
Posts: 10044
Loc: MA, Mittelfranken.de
Quote:
Good gas mileage as always - Not the upstream O2 sensor

The problem with upstream O2 sensors is there is one of them or two if its a V engine (generally speaking, posters please don't bombard the tread with odd balls that may use 4 or 1 for each double bank).

This means that if for example 1 cylinder is getting more fuel the ECM with lean all the others on that bank down to achieve an average as close to stoichiometric as possible. Basic overall fuel delivery amount will be the same but the 3 other cylinders will actually be running somewhat lean. This on going state of confusion can cause issues in other parts of the system.
On the other side if one cylinder is lean for some reason the ECM with richen the others. Again because one is less and the others are richer the average fuel deliver amount remains the same.

The only way to prevent this averaging would be to use an upstream and a downstream sensor for each individual cylinder.

Quote:
No hesitation/engine runs strong - Not the Cat


True if the cat was plugged otherwise no. If the wash coat is damaged or gone but the cat is not plugged it will not effect performance.

Quote:
Downstream O2 sensor is failing or failed. Does this seem correct?

The downstream sensor looks for a difference from the upstream sensor therefore verifying catalytic action took place.
In principal the closer the numbers are to each other the less efficient the cat is.

A weak front sensor may not throw a code but may not be signalling reliable information.
For this reason i said a comprehensive view using a scan tool is important for correct diagnosis. Throwing parts at an issue like this is a mugs game at best.
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#2437401 - 11/17/11 03:08 PM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
Vikas Offline


Registered: 07/22/05
Posts: 8265
Loc: NorthEast
Computer creates (or observes) condition under which it knows that cat has to "work". Typical condition is when you take the feet off the gas pedal while driving at highway speeds. If other conditions are "correct", then computer decides to "test" you catalytic converter. It compares cross counts (number of swings of the O2 sensor voltage from high to low to high etc) of front (i.e. before the cat) and rear (after the cat) sensors. It wants to see lot less (i.e. 10 times less) on the rear sensor than on the front. Now it knows that your cat is doing its job. It will start throwing the code if instead of 10 times less it is only 9 times less. HOWEVER, the actual emission will still way below the federal limits IF your car itself is in good tune.

When O2 sensor starts failing, it will NEVER (well, almost never) become faster i.e. more cross counts. It usually becomes sluggish. That is why very rarely rear sensor replacement is ever a cure for P0420/P0430.

Quest; can you explain how the freeze frame data is going to be useful in case of P0420/P0430 ? The fuel trim numbers do not trip that code. The cross counts are not in the freeze frame. If your scanner can do live data or better, graphs, you have a chance of catching them in the act but it will not be easy unless your car supports newer faster refresh protocol along with the fast scanner. Older protocol such as the ISO update speed is too slow to get good O2 switching graphs.


Edited by Vikas (11/17/11 03:14 PM)

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#2437440 - 11/17/11 03:47 PM Re: P0420 Code... CAT or O2 sensors? [Re: MysticGold04]
SteveSRT8 Offline


Registered: 10/10/08
Posts: 15175
Loc: Sunny Florida
Just take that rear one out and pack it in some steel wool.

Works sometimes.

We simply read the actual voltage at the sensor. If it's out of range replace it.
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#2437456 - 11/17/11 04:07 PM Re: P0420 Code... CAT or O2 sensors? [Re: Vikas]
Quest Offline


Registered: 12/19/04
Posts: 6427
Loc: beaver land EH?
Originally Posted By: Vikas
Computer creates (or observes) condition under which it knows that cat has to "work". Typical condition is when you take the feet off the gas pedal while driving at highway speeds. If other conditions are "correct", then computer decides to "test" you catalytic converter. It compares cross counts (number of swings of the O2 sensor voltage from high to low to high etc) of front (i.e. before the cat) and rear (after the cat) sensors. It wants to see lot less (i.e. 10 times less) on the rear sensor than on the front. Now it knows that your cat is doing its job. It will start throwing the code if instead of 10 times less it is only 9 times less. HOWEVER, the actual emission will still way below the federal limits IF your car itself is in good tune.

When O2 sensor starts failing, it will NEVER (well, almost never) become faster i.e. more cross counts. It usually becomes sluggish. That is why very rarely rear sensor replacement is ever a cure for P0420/P0430.

Quest; can you explain how the freeze frame data is going to be useful in case of P0420/P0430 ? The fuel trim numbers do not trip that code. The cross counts are not in the freeze frame. If your scanner can do live data or better, graphs, you have a chance of catching them in the act but it will not be easy unless your car supports newer faster refresh protocol along with the fast scanner. Older protocol such as the ISO update speed is too slow to get good O2 switching graphs.


@ Vikas,

Your observation RE: rear 02 sensor replacement rarely cures the P-0420 issue, and if that O2 sensor ever throws you a code is noted; it's typically complains about O2 efficiency below threshold and nothing more.

I understand that the crosscount will not become faster.
My current OBDII scanner is basically a scangauge on-loan to me to deal with anything circa 2002 or newer (older one, from 96 ~around 2003 are handled by my Equus OBD-II scanner). I figure that the newer ones may be able to report the fuel-trims, etc. in real time mode so as to report the situation better, and my focus of the obeservation is still on the front O2 sensor part in relation to that to the other variables, not so much as to reporting on the post-cat O2's efficiency.

maybe a freezeframe capable OBD-II scanner isn't all that useful in dealing with this subject matter, afterall.


To really watch how well the front (wideband O2) sensor behaves is to have it rigged to an oscilloscope, and run the engine, something that may be beyound most home/DIYer based auto repair.

My next major spending would be a computer (laptop) based OBD-II scanner interface with the ability to monitor various different variables "live" and infinitely code upgradable. That would helps in diagnosing a lot of difficult to repair problems revolving OBD-II systems.
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