P0420 comes back after CAT and O2 sensors replaced

The OP did have the after cat sensor replaced. Also, the code is not telling you to check the after cat sensor. It is telling you to check the cat!!!!!

Here is an analogy to help you understand how P0420 is generated: Think of the cat as a muffler, and the O2 sensors are microphones installed before and after the muffler. A microphone generates voltage as the magnet moves back and forth, and an O2 sensor generated voltage in the presence of oxygen. In the example, mike #1 is measuring sound, the sound gets attenuated in the muffler so the sound level coming out of the muffler should be quieter than what is going into the muffler. If this is the case, mike #2 should not pick up as much sound as mike #1. If mike #2 picks up the same sound as mike #1, the muffler isn't working and so P0420 is set. Jeff's assertion that if O2 sensor #2 is picking up a signal, it more likely to be bad than the cat being bad is like saying that if mike #2 is picking up sound, it is more likely that mike #2 is bad than the muffler being bad.

Here is another analogy along the same lines... a librarian who is deaf has to enforce a no-nose rule in a closed room with no windows. Since the librarian cannot hear, they install a microphone in the room and hook it up to a sound meter. By looking at the meter when the room is empty, the librarian knows the level of background noise when the nobody is making noise. If the librarian sees the sound meter moving, then using Jeff's logic, then the microphone must be bad because it is picking up sound. What is more likely? That the mike is bad because it is picking up sound or that there is noise in the room? Under the logic programmed into a car's computer, if sensor #2 is seeing activity, it will set P0420.

Since I posted previously that multiple re-occurrences of P0420 seems to be common in Corollas, there may be another explanation for the code and I recommended that the OP have it checked out at a dealership. Perhaps the threshold for turning on the MIL is too low?

Ray O (correct punctuation to reply)

Reply to
Ray O
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Does that mean the remaining 3% are concentrated around Phoenix, like the Ford dealer that thought noise from my speedo cable was caused by a faulty accessory belt or tensioner (even though the noise varied with the speed of the vehicle, not the speed of the engine) or loose wheel covers (they were loose -- ten miles away in my garage).

Reply to
larry moe 'n curly

Yes, that is precisely what the code is saying. I AM SAYING THE CAT IS FINE, therefore the sensor is the problem.

And, I see that he says the sensor was replaced ...

Reply to
Jeff Strickland

I get HOW the sensor work, and what they are telling us. What I am saying is that the after CAT sensor is saying the CAT is not working right, but I maintain the CAT is fine and the after-CAT sensor itself is failing.

The exhaust stream going into the CAT is okay, but the stream coming out is being reported as out of spec. If the stream is REPORTED as out of spec, then the problem can be eitherb the CAT itself, or the sensor making the report. The stream going into the CAT is within spec, therefore there is no point in checking system components that adjust the upstream exhaust contents.

If what goes in is okay, but what comes out is wrong, thent he CAT itself is suspect, or the sensor making the report is bad. Since the CAT is reliable and the sensor is not, I'd replace a $50 sensor long before I'd replace a $200+ CAT. In any case, I would not be looking to components that make adjustments to the fuel mixture going into the engine.

I'd suggest further that the firmware itself is problematic if what you say about Corollas is true. If the firmware is looking for a condition to exist in a certain timeslice but the timeslice is too small, then false reports of underacheivement of the spec can be expected even if the spec is met over a longer period of time. I've read reports of recalls specifically for this sort of thing -- but admittedly I do not recall if those reports are for Toyotas/Corollas or other makes/models.

Reply to
Jeff Strickland

All the dealerships I called on sold Toyotas, only a half-dozen or so also sold Fords. I don't consider a sample size of a half-dozen to be large enough to be able to take a guess as to their honesty and competency.

Reply to
Ray O

Just to add fuel to the bad firmware theory, have a friend who had this problem on '02 Corolla. Chk light was reset & drove the vehicle 7 to 10 days to allow the computer recycle then took it for emission inspection & passed with flying colors. If in fact the CAT or the mixture being emited was actually bad would not the vehicle failed emissions?? Chk Eng light has come on since & been reset & stays off for several weeks or months before it comes on again. Am certainly no expert but this intermitent behavior certainly sounds like some sort of false\positive &\or computer problem. dc

Reply to
doncee

The facts don't match what you maintain, therefore you ignore the facts?

How do you KNOW that the exhaust going into the cat is within spec?

You may think that there is no point in checking system components that adjust the upstream exhaust components, however, the folks that designed and built the system think differently and so list several components and systems to check. I would trust their advice before I trusted your advice or my advice. I am simply relaying what the factory says to check.

If you have no idea what you're doing and are willing to replace parts without knowing whether they are good or bad, then replacing cheap parts before expensive ones has some merit. This is where we differ - IMO, checking an O2 sensor is so simple, taking less than 5 minutes, that it is worth checking it before replacing it. Unplug the sensor, hook up a volt meter, look at the meter for 1 minute.

P0420 is not set because of a short timeslice or underachievement of a spec. This is an over-simplification but the computer wants to see what looks like a sine wave from sensor 1, with the peaks and troughs reaching a certain height, and then it wants to see the wave from sensor 2 to be much flatter, with much lower peaks and troughs. If sensor 2 has high peaks and troughs that match what sensor 1 is reporting, then P0420 is set. In other words, the computer is looking for the absence of activity from sensor 2 and if there is activity, then P0420 is set. Since there is always some activity from sensor 2, then the "alarm" threshold may be to low. If a firmware upgrade is needed, it would most likely involve reducing the maximum voltage threshold from sensor 2.

Reply to
Ray O

No, because emissions checks on vehicles with OBD II do not actually sample the emissions. An emissions check on an OBD II vehicle involves plugging a code reader into the OBD II port and reading the contents of the vehicle's computer to make sure that it is ready and has not detected a problem for the past 2 drive cycles. If the vehicle has been driven for 2 drive cycles without detecting an error, then it is "ready," and will pass the emissions check.

Chk Eng light has come on since

Most diagnostic trouble codes (DTC) have 2-trip detection logic, where the electronic control module (ECM or computer) has to detect a problem on 2 consecutive drive cycles, or trips. If the ECM does not detect the problem on 2 consecutive trips, it will turn the malfunction indicator light (MIL or check engine light) off. A marginal problem can make the MIL go on and off.

Reply to
Ray O

Because there is a sensor that says so. More accurately, there is a sensor that does not say the exhaust stream is out of spec.

I would trust them too, but in the face of not having all of the diagnostic tools at my disposal, I have to replace stuff at home using a Best Guess method. Since there are sensors before the CAT that are not giving an alarm, and there is a sensor after the CAT that is, AND I have faith that the CAT is a reliable component, I have to give my Best Guess on a failed sensor afgter the CAT. I would replace the sensor that is giving the alarm and hope the alarm goes off.

Reply to
Jeff Strickland

I don't know where you live, but in California, they put a probe in the exhaust pipe to actually measure the exhaust stream coming out. They do other stuff too, but they ALWAYS measure the exhaust output at the tailpipe using a probe.

Reply to
Jeff Strickland

Which sensor checks whether the exhaust stream going into the cat is in or out of spec? What is it checking for?

If one is going to attempt carpentry, one should have basic carpentry tools on hand, including a tape measure to measure length. With a tape measure, a carpenter can measure length instead of cutting boards until one fits. If one is going to attempt automotive diagnostics, one should have basic automotive tools on hand, including a volt/ohm meter to measure volts and ohms. A perfectly servicable volt/ohm meter can be purchased at Radio Shack for under $25. With a volt/ohm meter, one can measure voltage instead of replacing parts until one works. That $25 investment in a volt/ohm meter will save hundreds or even thousands of dollars by avoiding replacing good parts. In my humble opinion, spending $25 on a meter that can be used for checking just about every electrical component in a car is a reasonable investment to help decide whether one should spend $50 on an aftermarket O2 sensor or even a $125 OEM sensor. Not only will you know whether or not the sensor needs replacement, you will have a tool that will help check other stuff in the future. I spent around $15 for my meter at Radio Shack almost

30 years ago, and as much as I would like a $400 Fluke meter, my cheapo meter has never not worked for checking anything in a car.

Please think about and answer this a hypothetical question: You have 2 different O2 sensors, one in a blue car and one in a red car. The one in the blue car is putting out a signal that meets factory specifications and is rapidly cycling between 200 and 800 mV. The one in the red car is putting out the same signal as the one in the blue car. Is the sensor in the blue car good or bad?

To check your knowledge of how sensor #2 works, go to this link for on-line test questions for the ASE test

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and do questions #7 and #17. The site lets you know whether your answers are correct or not. In the scenario described in question #7, DTC P0420 would be set. I don't expect everyone to know and understand the answer to question #7, so in my desire to share my experience with and teach those who do not know, I gave the CORRECT answer. My answer happens to agree with the National Institute for Automotive Service Excellence, which is the body that tests and certifies automotive technicians, with the factory serive manual, and I suspect with Haynes, Chilton, and anyone else who actually understands how DTC P0420 is set. Why do you insist in giving advice that is contrary to the advice given by the factory and every legitimate source of automotive information?

Reply to
Ray O

I guess the only answer is to get you two a set of gloves...LOL

I have to admit, I'm learning from both of you and grinnin a bit along the way....

Reply to
Scott in Florida

Miss placed faith. The OP has already stated that he did NOT opt for a genuine OEM Toyota cat. Fact; original equipment OBD2 certified catalytic convertors have to by EPA rules, test at better than 75% efficiency. Aftermarket catalytic convertors only test to 40% efficiency and often times, a fair amount less.

The problem the OP is having is well known and discussed in the emissions industry, that being; most aftermarket cats are no where near efficient enough to meet the OBD2 monitoring criteria, and in almost all cases, result in a failed repair which is usually followed by mass parts swapping based on the flawed logic that the new aftermarket cat -must- be doing its job so there

-must- be another failed component.

Reply to
aarcuda69062

Before replacing an O2 sensor, I would read their voltages, either directly or through the car's computer. You'll need either an oscilloscope (for electronics, not the kind for ignition systems) or a digital voltage meter (don't use the analog type since it will load down the sensor too much and slightly damage it). It's best if the meter has a bar graph because that reacts much faster than the digits display. The front O2 sensor should read about 1/2 volt AC on the meter, or about 1 volt peak-peak on the oscilloscope. The rear sensor should read the same if the catalytic converter doesn't work at all; otherwise its voltage reading will be much lower, about 0.1V peak-peak. You may be able to do a very rough check of the catalytic converter by pointing an infrared thermometer at it. It should be hotter than the exhaust pipe in front of it by at least tens of degrees.

Reply to
do_not_spam_me

LOL! You are learning from whoever is right and being confused by whoever is wrong!

This discussion is a lot like ones my wife and I have with my daughter ;-)

Reply to
Ray O

Agreed. My point is that he should never have replaced the CAT in the first place. He had this code several weeks ago, and my suggestion then was to replace only the after-CAT sensor. He elected to follow the advice of another poster and spend more money than I would have spent to replace what is normally a life item.

If I was getting a P0420 code, I would reset the computer and wait for another occurance. If it came, then I'd replace the after-CAT sensor and reset the computer again. If the code repeated at that point, then I'd learn the troubleshooting procedure for the various sensors and wiring harness.

Now that the CAT has been replaced with an aftermarket part, all bets are off. I agree that the new CAT might not work well, and if the sensor he installed required him to splice in a new connector, then I'd suspect that repair as well. In my humble opinion, he had a relatively easy problem that has been blown out of proportion. I can't say with any certainty because I am on the other side of the computer screen, but my gut feeling is that he had a failed sensor that could have been replaced in less than 5 minutes, but a different approach was taken. Now, I think he needs professional help with good parts and diagnostic equipment.

Reply to
Jeff Strickland

California does BOTH - first the Testing Computer scans the Car's ECM Computer and looks for either the AOK code that the engine has completed two complete drive cycles successfully with no codes set (which won't be there if the car ECM was reset just before the test in an attempt to try sneaking by) or for any trouble codes that have been set.

If it fails that part, there's no sense going any further.

After that step is passed they call for a visual check for tampering with under hood systems or hacking out cat converters, tampered evaporative emissions systems, bad gas cap seals, etc.

And only after you pass all that THEN they call for the sniffer probe to be inserted in the tailpipe and a short drive cycle on a chassis dynamometer, to see if the ECM's idea of 'Normal' meets with reality.

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Reply to
Bruce L. Bergman

Yeah, Scott - and since Ray seems to be the one who actually knows WTF he's talking about here, he gets the good gloves. (The ones with the horseshoes sewn in.) ;-)

Jeff: Stop arguing and talking nonsense and listen to Ray, he has the training on this stuff. His ASE Certs may have expired, but that doesn't mean he's suddenly turned into an blithering idiot.

(Several other things could explain that, but I digress...) ;-)

Catalytic converters are reliable, but only to a point - they do wear out over time and from accumulated use. You are using the old American Car meme where a car is "worn out and ready to scrap, and buy a new one at 100,000 miles", and thinking that way yes it's a "lifetime" item.

And while a catalyst should make it that far, after 100K it's a crapshoot how much farther. It dies, you replace it and keep going.

And they also get poisoned by lead and other heavy metals that can get into the system. Coolant leaks, use of the wrong "carburetor" (throttle body) cleaning solvents or improper sealants that get through and poison the Platinum and other noble metals in the matrix.

And they can have a partial ceramic matrix meltdown from too much unburned fuel (bad sparkplug or coil causing a misfire), creating a bypass passage that lets uncatalyzed hydrocarbons get around the matrix and out the tailpipe.

When the #2 Oxygen Sensor after the catalyst is making the same wide voltage swings as the #1 sensor before the catalyst, that's pretty reliably showing that the catalyst is not working.

If the catalyst is working properly you want to see big and more-or- less matching swings on both the #1 and #2 Oxygen sensors at a cold start as soon as the sensors warm up.

And soon after that the catalyst matrix warms up to operating temp and lights off, and starts burning up the excess hydrocarbons and pollutants. And while the #1 sensor will keep making bigger swings with throttle and load changes, you should only see corresponding

*small* voltage swings on the #2 sensor after that. Meaning the catalyst is scrubbing the gases properly before the #2 sensor.

If the #2 sensor is bad you'll see a very weak or flat-line signal all the time - cold, hot, full or part throttle, over-run down a hill, the output will be either a few faint twitches or nothing.

Now you have to determine WHY. Just throwing expensive replacement parts at the car until the light goes off (and it might never go out) without understanding /why/ you are doing it is a fool's game.

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Reply to
Bruce L. Bergman

LOL....I know who is right, but it is entertaining to watch the back/forth...

LOL....

Kids are that way....

Some arguments you don't/can't win....

Reply to
Scott in Florida

He had diagnostics from two different shops, one a Toyota dealership that indicated that the cat-con was no longer functioning. Catalytic convertors do not last forever, that is why you can buy replacements. Trick is to buy the -right- replacement.

If I had a code P0420, I'd use my scan tool to do some dynamic testing of the oxygen sensors involved. If I force the fuel system rich and lean and the oxygen sensors respond accordingly, then the sensors are functioning as they should be.

I believe that he mentioned in one of his earlier posts that he bought genuine Toyota sensors on line and saved $40 per sensor doing so.

I think that originally, he got ambiguous advice from the Firestone store which then planted the seed of doubt when he went to the dealership. At 150K miles, I wouldn't want to give any warranty on a job like this when there's a good chance that the O2 sensors are original or very high mileage, so the recommendation that both be replaced is sound business management.

There are plenty of trouble codes that will identify a stuck high, stuck low, lazy, open, or shorted O2 sensor, hell, Nissans have their share of post cat o2 sensor failures, but they don't flag a P0420 when it happens.

Reply to
aarcuda69062

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