Re: P0420 code and PA state inspection

I have P0420 faulty code issue for over 6 months, and now it is the

> time for PA state inspection. I talked to some guys at Firestone auto > shop and was told that both O2 sensors need to be replaced along with > catalytic converter in order to ensure catalytic converter warranty. I > tried to back-probe the post O2 sensor for my 2000 Corolla with 150K > miles on it, but failed(Followed the auto repair book, but cannot id > anything, maybe it is too complicated for me). Firestone asked $100 for > computer diagnose for the problem and they also said that if post O2 > sensor was bad, they would not know whether catalytic converter was > good or not because of misleading signal given by post O2 sensor. > Basically they cannot give positive conclusion on whether post O2 is > good or not by running computer diagnosis. I have it diagnosed at > Toyota dealer 6 months ago and when I tried to retrieve diagnose info > from dealer, what they have is the suggestion to replace both O2 sensor > and catalytic converter(at that time, I had no idea O2 sensor can cause > P0420 faulty code) at $1500. > Here are my questions: > Is it necessary to replace both O2 sensors for bad catalytic converter? > > It seems to me that I run out options but go ahead to have both O2 > sensors and catalytic converter replaced. Any other suggestions for > what I should do? > > Thx >

It is a common procedure to replace one or both O2 sensors when P0420 comes up, however, it is possible to check the signal output from O2 sensors and determine whether they are functioning correctly or not, and with an exhaust gas analyzer, it is possible to check catalytic converter performance. The Toyota dealer should have the equipment to analyze both the O2 sensor signal and the exhaust gas.

Reply to
Ray O
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Thanks very much Ray. I have it done at dealer 6 months back and when I went back to delaer this morning and found out what they could tell me is that the suggestion to replace O2 sensor + catalytic converter. They asked me to re-do the analysis since over 6-month passed. I don't like to pay the same service again since it will cost me another $100 and may draw the same conclusion. Here is what dealer found out at the first time: "Access ECU, Locate DTC P0420 for catalyst below efficiency Diagnose catalytic converter internally worn new catalytic converter and oxygen sensor installed $1530.78 plus tax Recommendation: Front motor mount worn $219.55"

They claimed that "catalytic converter internally worn" and I have no idea how they identify it through exhaust gas analyzer(honestly, I am not sure whether they run the gas analyzer or not since it only costs me $56 for checkup)? I am not sure whether I should trust them or not. I prefer not to spend another $100 to do diagnosis, I may put it into replacement instead. Ray, in your opinion, what is the cost effective way to do it? Thanks.

Reply to
wenmang

I just got off phone with dealer rep and he said that they used a scanner tool to scan the output of O2 sensor and catalytic converter instead of using exhause gas analyzer. He told me that the scanner does more than just reading OBDII code and it can pin point the problem. The conclusion is that the scanner tells them both of post O2 and cat converter are bad.

Reply to
wenmang

Most modern service tracking software requires the service advisor to enter a CCR, or Condition (what the customer is saying or asking), a Cause (what is causing the condition that the customer is describing, and a Remedy (what the technician did to fix the problem).

The technician that looked at the car did not write a good explanation of what he found, and rather than going back to ask the tech, the service advisor made up the cause. I know this because catalytic converters do not "wear internally." By definition, a catalyst is something that causes a chemical change without the catalyst itself changing or being used up, so in theory, the catalyst in a catalytic converter will last forever. In reality, the particulates (soot) in the exhaust gradually form a coating on the catalyst so that the exhaust gas no longer comes in contact with the catalyst. The other thing that can happen to a catalytic converter is that the metal structure corrodes and the corrosion blocks the exhaust flow or coats the catalyst.

My guess is that the both the dealership and Firestone did not actually check O2 sensor performance and did not do an analysis of the exhaust gas. They pulled the code and recommended that you change everything that could cause that code, which is the 2 O2 sensors and the cat itself.

With that shotgun approach, fixing the problem is a pretty sure bet, but personally, I would prefer to actually check O2 sensor performance before replacing it. If you are only want to replace one O2 sensor and want to take a shot at which O2 sensor to replace, replace upstream one in the exhaust manifold.

If you just want to pass PA emissions, just change the cat. Keep in mind that after the work is done, it can take up to a week for the computer to reach "ready" state.

Reply to
Ray O

You can go with what they told you and be pretty sure the problem will be fixed, or if you are willing to gamble a little, you can go with just the Cat and see what other trouble codes, if any come up. The downfall to this approach is that you may have to make another trip and pay another diagnosis, but the up side is that you won't pay for unnecessary parts.

Here is basically how the O2 sensors and catalytic converter (cat) work:

There is an O2 sensor in the exhaust ahead (upstream, or #1) of the cat, and one after the cat (downstream, or #2). Sensor #1 measures oxygen in the air, and from that measurement, can determine if the air/fuel mixture has too much or too little fuel. Sensor #1 tells the computer what to do, the computer makes an adjustment, and then Sensor #1 takes another measurement to determine if more adjustment is necessary. This happens very quickly, so the signal coming from the O2 sensor is constantly fluctuating.

After the exhaust gas flows past Sensor #1, it enters the cat and gets cleaned up, and then flows past Sensor #2. If the cat is working properly, the signal from Sensor #2 should not fluctuate. If the signal fluctuates, then the computer assumes that the cat is not working and stores trouble code P0420.

If sensor #2 were not working, then it wouldn't know the cat is bad or it would set a different trouble code.

In other words, the code scanner does not measure the exhaust gas directly; it looks at readings from various sensors and infers whether the exhaust is clean or not.

Reply to
Ray O

Thanks Ray.

After reading your post, I've decided to have the whole things replaced. In past over 10-months(not 6, sorry), I keep cancelling the code(P0420). The faulty code used to come up within 2-month. But recently, it keeps showing up every 2 weeks(no other code except P0420, pretty consistant). I may assume that post O2 sensor may be good. Local shop will not provide warranty on Cat unless 2 O2 sensors are replaced at the same time. Either way, I have to pass PA inspection soon. Anyway, thanks for the education. WM

Reply to
wenmang

You're welcome, good luck!

Reply to
Ray O

Ray,

You gotta help me out here. I understand before- and after- CAT codes, but I think you have them wrong. Bank 1 and Bank 2 refer to the left and right banks of a V motor, V6, V8, V10, or V12.

I refer you to the P0100 ~ P0199 series of codes. Specifically, P0130 and P0136 refer to Bank 1 Sensor 1 and Bank 1 Sensor 2, respectively. Not that further down the list is Bank 1 Sensor 3, Eventually we find that P0150 is Bank 2 Sensor 1, and the pattern repeats itself with Sensor 2 and Sensor 3.

The OP said his car reported P0420, which is CAT system, Bank 1. P0430 is the same error, except it is on Bank 2. This would be a Dual Exhaust equipped car or truck. Bank 1 is the bank that shares the #1 Cylinder, and Bank 2 is the bank other than the bank with the #1 Cylinder -- since some motors have sequential numbers, ie 1 ~ 4 on one side and 5 ~ 8 on the other, and others all have odd numbers on one side and even on the other, all we know is that Bank 1 is where the #1 cylinder is, and Bank 2 is where the #1 cylinder isn't.

I'm at a loss to explain Sensor 1, Sensor 2, Sensor 3 -- as are shown in the P0100 Series of codes -- but I hazard a guess that these would limited to performance motors but I really am not sure of that. I've never owned a car that had more than 2 sensors before the CAT and 1 after, but my V8 and V6 motors have single exhaust, so I get P0120, P0150, and P0420 depending on what is unhappy.

Reply to
Jeff Strickland

Bank 1 and Bank 2 does not refer to the left and right banks. Bank 1 refers to the bank that contains cylinder #1 and Bank 2 refers to the bank does not contain cylinder #1 in the banks of a V, W, or horizontally opposed engine. Obviously, an in line engine configuration contains only 1 bank so there is no bank 2 in those engines.

The vehicle does not necessarily have a dual exhaust. For example a 6 cylinder Camry has a single tail pipe.

For a V6 Camry, each bank has an exhaust manifold, where sensor #1 is located, then the exhaust manifold leads directly to the cat, then there a pipe that leads to a Y junction, then through the single exhaust out the back of the car.

Bank 1 is the bank that shares the #1 Cylinder, and

Some vehicles have multiple cats - in other words, instead of 2 cats on a V configuration motor, it has 4 cats. Sensor #3 monitors the second cat for a particular bank.

Reply to
Ray O

Okay, I get that. The V configuration meets the same definition and is more prevelent than the boxer engines.

You seemed to suggest earlier that Bank 1 and Bank 2 was either before or after the CAT. The OP clearly has a code that describes the exhaust gasses coming out of the CAT are not within specs, although the gasses entering the CAT are okay -- assuming the lack of a code means nominal operation.

I understand that. But the OP specifically said that he had code P0420, which only has Bank 1.

I get all of that, which brings us to my confusion. You said -- or I read that you said -- that P0420 was indicating he needs O2 Sensors. I suggest that all he needs is the after-CAT sensor, and that the O2 Sensors would throw a different code if they were suspect.

Clearly the code he brought us says he needs a CAT, but we both kno that the CAT itself is very reliable and the most likely cause of failure is feeding it Leaded gas, and leaded gas in not even available for purchase anymore. Sure, there are a few other things that can happen to a CAT, but he would be reporting drivability problems and a code, not just a code. This means that all he needs is a new sensor as the most likely problem spot. If the new sensor does not solve the problem, then he needs to dive into troubleshooting the problem.

Bank 1 in the P0420 Code would mean the after-CAT sensor in the exhaust system linked to Cylinder 1. P0430 would mean the same thing in the other exhaust system in a dual exhaust system, if the vehicle had one.

Okay, I was forgetting that. I've seen several cars with two CATs. I've not seen one with three, but I follow the logic.

Reply to
Jeff Strickland

By reading your post, I am more confused. If P0420 is there, is post O2 sensor is good or bad? P0420 keeps coming back even I keep canceling the code. If CAT is not easy go bad, what went bad then? Since only P0420 is keeping generated, I guess that post O2 sensor must be good, right?

Reply to
wenmang

The distinction of how to identify which bank is bank 1 is important, especially if one recommends changing parts without checking them, kind of like a doctor performing knee surgery on the wrong knee.

When I refer to sensor #1 or #2 (not Bank 1 or Bank 2), I'm referring to the sensors located before and after the cat. The bank is irrelevant, other than pointing to the side that needs checking.

The OP clearly has a code that describes the exhaust gasses

That is my point. Why are you recommending that the OP change the the after-cat sensor?

P0420 and P0430 does not mean that a vehicle has one or 2 banks. Those codes point to the bank that needs to be checked.

I would not say or indicate that P0420 indicates that the vehicle needs O2 sensors because that is an incorrect interpretation of the DTC. - YOU are the one suggesting that the OP needs sensors. The shop the OP took his vehicle to is suggesting that the sensors should be replaced along with the cat so that the replacement cat's warranty is valid. IMO, that is a way for the shop to avoid responsibility for a mis-diagnosis.

I suggest

This suggestion makes no logical sense. You suggest that the OP needs an after-cat sensor even though the trouble code does not suggest that the OP needs one.

What I know is not the same as what you know. Leaded gas is not the most likely cause of failure. IMO, the most likely cause of failure is deposits that coat the catalyst and prevent the exhaust gas from coming in contact with the catalyst. Those deposits can come from particulates, oil consumption, or aftermarket fuel or oil additives, or from exhaust manifold or pipe corrosion.

If the catalyst is coated, there will not be any effect on driveability.

This means that

Your base assumptions are not correct, so the following logic and repair recommendations are not correct.

No, P0420 means that the cat is operating below threshold, not that the O2 sensor is bad. Go back and read what I have posted several times about how you cannot get a false bad reading of the exhaust gas.

P0430 would mean the same thing in the other

No, you can get a P0430 in a vehicle with 2 banks and a single exhaust system.

Did you read any of my posts explaining how the system works?

Modern Toyotas with V6 and V8 engines have two cats, one for each bank. The cat is not under the car in front of the muffler. The cat is attached to the exhaust manifold before the crossover pipe so you have 2 cats, 1 exhaust pipe.

Reply to
Ray O

You are confused because someone is posting incorrect information and bad advice without knowing how the system works.

If P0420 keeps coming back, the most likely cause is a bad catalytic converter. It *may* still be covered by the emissions warranty. It is possible that the O2 sensor before the catalytic converter is operating marginally, and this can be checked at the dealership. If P0420 comes up, yes, the post o2 sensor is most likely good.

Reply to
Ray O

I made the distinction.

But, YOU refered to Bank 1 and Bank 2 as before or after the cat. That is incorrect. Bank 1 and Bank 2 refer to the banks of cylinders, not the location in the exhaust stream.

The OP has a code that is Bank 1, but YOU continue to insist it is an O2 Sensor located in front of the CAT.

Because that is what the code is telling him to do.

P0420 is Bank 1 sensor, P0430 is the same sensor, except it is Bank 2.

Excuse me, but that is precisely what YOU suggested, which is what prompted me to ask my question.

The OP needs a new after CAT sensor because odds favor a failed sensor ahead of a failed CAT, and the code he reported (P0420) says the CAT is not functioning. My _guess_ is that the CAT is functioning fine, and he needs a new sensor, but not an O2 sensor that lives in the exhaust system ahead of the CAT.

The shop the OP took his

That is precisely what the code suggests.

I'll give you additives, but the other stuff will not exist in such a young car.

Leaded gas will destroy a CAT in a tankful, but I do not know of anyplace that still sells leaded gas, so we can discount that as a likely problem area. Additives could be a problem source

No you can't.

Please give me the code for an after-CAT sensor in a dual exhaust car. For that matter, give me the code for the after-CAT sensor in a single exhaust.

Yes, I know that. And each cat has its own after-CAT sensor, which explains the P0420 and P0430.

You keep saying stuff without any reference for me to look up. Please give a cite for the sensors and the respective codes.

Reply to
Jeff Strickland

If I referred to Bank 1 or Bank 2 as before or after the cat, then that was incorrect. I meant sensor #1 and sensor #2.

I think you are reading someone else's post and mistaking it for my post. I did suggest checking the sensors in front and after the cat.

DTC P0420 is Bank 1 catalyst system efficiency below threshold. Nothing in DTC P0420 is telling anyone to change anyone to change any sensors, and in fact, there are no DTC's that tell anyone to change any parts.

All a DTC does is point the technician to the component to CHECK, not CHANGE. In the case of DTC P0420, of P0430, the DTC indicates that a CHECK of the catalyst is in order, not a change of any sensors.

Since it only takes a few minutes to check an O2 sensor, a thorough technician should check the operation of the after cat O2 sensor, but I would not recommend assuming that is bad and changing it without checking it.

If you go back and look at my posts on various subjects, you will note that my advice is almost always to CHECK this or that component. Checking before replacing is something that was ingrained in me by some of the best automotive diagnosticians in the world, and it is something that I preached and enforced with the dealerships I was responsible for.

I think we're saying the same thing, only differently, here.

I never suggest that any trouble code indicates that anything needs to be replaced. Perhaps, when I said **CHECK** you are interpreting that as "replace." I did say that the Bank 1, sensor 1 and bank 1, sensor 2 should be CHECKED, along with checking catalyst performance by probing the exhaust stream upstream and downstream of the catalytic converter with an exhaust gas analyzer probe.

Without an exhaust gas analyzer, it is possible to read the signal from the upstream and downstream sensors to determine whethere they are working, and if both sensors are functioning correctly and if sensor #2 is putting out the same signal as sensor #1, then the cat is bad.

You have me thinking, so I took a look at a '97 Avalon factory service manual, and the procedure in the book for DTC P0420 says to CHECK the operation of the Bank 1 sensor 1 and bank 1 sensor 2, and if they are good, to replace the cat. It does not say to replace sensor #2 and if the code comes back, then replace the cat.

I am not suggesting that your guess is incorrect. I am suggesting that the process you used to arrive at the conclusion is incorrect and can lead to unnecessary replacement of parts. As a factory service rep, if I were reviewing this claim if this was a warrantable repair and replacing #2 O2 sensor didn't cure the problem, the dealership's claim would be denied, or if the O2 sensor was inspected and found to be good, the dealership's claim would be charged back and the dealership would have to eat the repair.

This is kind of like a math class where merely coming up with the right answer won't get you credit unless you show the steps you used to arrive at the answer. My process is kind of "old school," but it allowed me to fix a lot of cars that had stumped dealership service departments.

We're going in circles here ;-)

It is also possible that the coating on the catalyst bed was too thin and physically wore off, or that the cat was marginal and finally failed.

The codes that refer to an after cat O2 sensor in a dual exhaust car are: P0136 O2 Sensor Circuit Malfunction (Bank 1 Sensor 2) P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) P0139 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) P0140 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2) P0141 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2) P0156 O2 Sensor Circuit Malfunction (Bank 2 Sensor 2) P0157 O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2) P0158 O2 Sensor Circuit High Voltage (Bank 2 Sensor 2) P0159 O2 Sensor Circuit Slow Response (Bank 2 Sensor 2) P0160 O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 2) P0161 O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2)

Trick answer: The codes that refer to an after cat O2 sensor in a single exhaust car are the same as the ones listed above because sensor 1, the cat, and sensor #2 are located upstream of the Y-pipe that joins Bank 1 and Bank

2 and leads to the muffler and tailpipe.

On a dual exhaust car, there is no Y pipe and each bank goes to a separate muffler and tailpipe.

In other words, "single" or "dual" exhaust refers to whether you have 1 or 2 tailpipes sticking out of the back of the car, regardless of whether the engine has 1 or 2 banks. A V6 engine can have 2 cats, one for each bank, and a single exhaust.

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Your teachers must have loved having you in class because you force them to really think the lesson through! There is the adage that one learns by teaching ;-)

Reply to
Ray O

That is correct. Tehre are no codes that tell anybody to do a damn thing.

I'm telling you, and the OP, that P0420 specifically says that EITHER the CAT is malfunctioning OR the sensor itself is malfunctioning.

The CAT certainly can operate below threshold, but most CATs cost in the range of several hundred dollars, and the vast majority of CAT problems I've ever experienced included driveability issues.

The sensor that reports the CAT is below threshold can and does fail, with alarming regularity. I've replaced a few sensors in my fleet of cars and trucks, and I typically buy Bosch parts for about $50. Given the fact that the sensor costs $50 and the CAT costs $Hundreds, I will replace a sensor ahead of a CAT any day of the week, and twice on Sunday.

If the exhaust stream is out of spec ahead of the CAT, a complete different set of codes will come up, but the OP did not report any of those codes.

Since there are no reported codes that come from upstream of the CAT, and the code we have says the CAT is not working, and the CAT is so expensive, I put my dime on the sensor giving a false report. Replace the sensor, or visit a stealership and ask them to bend you over the fender.

Precisely, but the OP is not a technician. He hasn't the capazcity to test anything, and was asking for pointers to get to the bottom of his problem from those of us that have experienced his pain on our own. I'm sharing my experience -- replace the damn sensor -- you are sending him to the dealership.

I agree, a thorough tech would do that. I would replace the part and cross my fingers. I have faith that the CAT itself is virtually industructable, especially when the vehicle is only 5 years old.

I understand. But the OP hasn't the capacity to do the check you advise. And, for my money, you advise checking parts that are not part of the problem. The parts you told him to check throw their own code if they are operating out of spec.

Whatever. You said that sensors that throw their own code should be investigated even though they did not throw a code. And, the OP hasn't the capacity to do the checks you said. I've got lots of stuff, and I can't do the checks you said.

Reply to
Jeff Strickland

Sorry, I hit the wrong button ...

Of course it says that. If I was writing a book targetted to factory trained techincians -- and I wrote just exactly that kind of stuff for 15 years -- I'd say that too. But, If I'm standing in line at the check out counter buying a new load of car parts, and the next guy in line is talking to the counter guy (who has a blank stare, like a deer in headlights), I'd not hesitate to suggest that the CAT is probably okay in an '00 Camry, and his code OUGHT to go away with a new sensor. I'd install a new sensor long before I would install a new CAT.

But the dealership can make the checks you think need to be done before they buy a new box of parts. I can't make those checks, so I have to have a certain amount of faith that the computer is not lying to me.

Well, I was also a factory rep, and I repaired lots of stuff that dealerships could not figure out. I, and I suppose you, have a pretty good instinct for this kind of stuff, but sometimes I have to defer to the built-in diagnostic system to tell me where to start.

I give.

Monkey's Uncle.

I readily admit that I assume the CAT works fine, and the "out of spec." report of the sensor is itself the malfunction. The sensor is malfunctioning and reporting a condition that does not exist. Replace the sensor, and the false report of a malfunctioning CAT will go away.

There are typos here ...

I listed these codes a few days ago (some of them) and said they were O2 Sensor Codes from BEFORE the CAT. Why didn't you correct my error when I made it? I would stopoped arguing the point several days ago.

The After-CAT sensor includes Catalytic Converter in the description of the code. None of these codes includes a mention of the CAT. As far as I know, the After-CAT sensor is not heated, only the Before-CAT sensor is heated, and it isn't headed in all cases.

Maybe this is where my error is ...

Reply to
Jeff Strickland

Where we differ is that I'd tell the guy to check the sensor before taking a shot and just replacinng it at $100 plus a pop, and if the code goes away, then take another shot with a $800 cat. As a factory rep, people would be seriously upset with the supposed "expert" who said to replace this non-returnable part and it didn't fix the problem and now who is going to pay for the unnecessary part?

The O2 sensor can be checked with a simple cheap

As I keep repeating, the built-in diagnostic system is pointing to the cat, not the O2 sensor. Go to this site:

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and scroll down to P0420 or P0430 and point out to me where those codes mention the O2 sensor.

From my experience with P0420 and P0430 in Toyotas, the cat was bad and the sensors were fine. I have not run across a false "bad cat" code in a Toyota yet. I think the reason for this is that a bad post-cat O2 sensor is more likely to think that a bad cat is good. IOW, when everything is working properly, the signal from the post-cat O2 sensor gives a pretty flat signal, which is what a marginally bad sensor is likely to put out so even if the sensor is marginal, the signals look the same as a good sensor and a good cat.

Because I was lazy and assumed you picked the right ones ;-)

I cut and pasted those codes from

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Here is another source of codes:
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that jives with the first site.As far as I know, there are no OBD II codes that list more than one component in the code description, i.e., cat and sensor, at least none of the codes listed in the factory service manual. Perhaps your source of OBD II codes also lists associated items to check when a particular code comes up instead of listing the SAE description?

Reply to
Ray O

I think the list of codes you are referring to is incorrect or whoever listed them added related things to check.

OBD II trouble codes all point to only one specific component. AFAIK, there are no OBD II trouble codes that list more than one component to check.

The only cat problems I have ever encountered that include driveability issues are GM ones where the cat is so clogged internally that it is restricting the exhaust, causing the same effect as a potato or bannana in the tailpipe. Otherwise, the cat is passive and whether it is working or not has absolutely no effect on driveability, fuel economy, or performance.

Did you get P0420 or P0136 through P0141? If the code was in the range of P0135 through P0141, then the chances that replacing the O2 sensor will cure the problem is probably 95% or better.

If you had P0420 and merely replacing a sensor cured the code, you need to play the lottery becuase the chances are the cat is bad.

I'll admit that my experience is almost exclusively with Toyotas, but the number of Toyotas I looked at was probably in the thousands.

It is my opinion that if someone is capable of locating and changing an O2 sensor him or herself, then that someone is capable of checking the O2 sensor. All you have to do is warm up the engine, unplug the sensor, stick a volt meter probe on the terminals in the sensor plug, have someone rev the engine over 2,000 RPM, and read the meter. In fact, checking the sensor is probably easier than trying to get the rusted sensor out of the exhaust without destroying the exhaust.

I had assumed, perhaps incorrectly, that the OP wanted to know what the proper procedure for diagnosis and repair of his DTC was. The shop that he had visited was taking the shotgun approach and replacing both sensors and the cat.

BTW, I've seen bad cats - my faith in their indestructableness is not as strong as yours. My LS 400 had P0420, the sensors tested good, and replacing the cat (fortunately, under the emissons warranty) fixed the problem.

Are you going to give the OP your money if the advice you give doesn't cure the problem?

Either your list of OBD codes is incorrect or you are interpreting them incorrectly.

You are objecting that I "said that sensors that throw their own code should be investigated even though they did not throw a code" yet you are advising the OP to change that sensor? I do not see the logic in this argument. BTW, the reason I said that those sensors should be investigated is because that is what the factory service manual says to do.

Reply to
Ray O

For the umpteenth time, P0420 is a code that says the CAT is operating below threshold, but if the CAT is reliable, THEN the problem must be a malfunctioning sensor.

If the stuff going into the CAT was below threshold, or above it, then there would be a code generated that specified the conditiion. Since there is not code for the stuff going in, but there is a code for stuff coming out, AND the CAT is a reliable piece of equipment that costs several hundred $$$ to replace, then the sensor that costs a fractiion of that is a good place to start. Especially at home under a tree.

They don't. My point is that the CAT is a reliable item and the sensor is inherently unreliable. Barring the ability to do a thorough check of the entire universe, a malfucntioning sensor is much more plausable than a malfunctiioning CAT.

My source says that P0420 is a CAT that is below spec. My limited experience is that the CAT is usually okay, and when it fails there are driveability problems to boot. The OP did not suggest any driveability problems, and only a sensor reading that is reports an out of spec condition.

I assume the condition is actually in spec, therefore the sensor making the report must be in error.

Reply to
Jeff Strickland

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