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How can I be sure catalytic converter really needed to be replaced?

My husband just paid $1900 to have the catalytic converter replaced in our 2005 Honda CRV with 202k miles. The check engine light appeared 2 days ago. At the dealership, the diagnostic gave P0420 code “internal fault with catalytic converter”. The service consultant explained that “over time, with age, contaminants build up in the converter and the expensive catalyst breaks down…converter heats up, turns red…if converter is not replaced, it can damage the engine.” Tomorrow I want to call the service station because after reading Car Talk and other websites, I’m not sure if the dealership did the proper diagnostics to really be sure the converter needed to be replaced. IE: did they first check the O2 sensors?? what really is the root problem that caused the converter to run hot?? What told them to replace the converter?? Does anyone have good questions to ask that can tell me if the dealership did the proper procedure? How can I be sure and what would tell me that they didn’t deal with the situation properly? Thanks for any comments you have!

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I was confused b/c at first I thought you had the catalytic converter replaced, then the light came (back?) on (still?) showing a P0420 code.

Or are you saying this is all one set of events in the last 2 days? The light came on, the code was read at the dealer and was P0420 and they replaced the cat.

So clarify which it is.

If the dealer has already replaced the cat then this is water under the bridge. In addition, if the engine light stays off then they were likely right about the cat.

As far as getting them to explain/verify whether they followed proper diagnostics, good luck. I halfway know what I’m talking about some of the time and I have a hard time with getting service folks & mechanics to lay it all out for me.

Anyway, the “root cause” of the code may or may not have been the converter getting hot. That code does not mean “internal fault with the cat.” All it actually means is that the readings from the upstream (before the cat) and downstream (after the cat) O2 sensors were too similar. The normal interpretation is that this is because the cat isn’t doing its job. However, one or both O2 sensors could have a problem. Or there could be an exhaust leak throwing off the sensor readings. There also could be a problem with the fuel/air mix that shows up in the O2 sensor readings.

I don’t have any earthly idea how you’ll get out of them what they did. But I would say that if the light comes back on I’d be telling them that for $1900 they’d better just get it fixed without charging any more.

Oxygen sensor codes start with a P01XX code. If a P01XX code doesn’t show up there’s no reason to suspect the oxygen sensors. The codes for the catalytic converter efficiency starts with P04XX codes. This a hard fault code.

When doing diagnostics on a vehicle you always start with the lowest code number that’s present. Because solving that first may eliminate the higher code numbers that are present. So if there were no P01XX codes there’s nothing wrong with the O2 sensors. But if there’s a hard fault code P04XX for catalytic converter efficiency, it means there’s a problem with the catalytic converter.


Rather than take the “high number/low number” code approach to this why not do the lab scope tests on the sensors and the inlet outlet temperature tests on the cat? OH I know why, because the work is already done.

I cannot see you approach the shop and say “explain to me why I should feel confident that the work you did was the work that was required to be done”. All questions about diagnostic techniques should have already been asked. You certainly don’t expect the shop to say something like “you know we just guessed at this one so here is half your money back”, do you?

Thank you to the three of you for your comments. All were very helpful and particularly helpful to learn that there are different P01XX codes for the O2 sensors. (to the first respondent, just to clarify - the engine light appeared Sunday morning and was on steadily thru Monday morning when my husband took the car to the dealership for servicing. Sorry for the confusion). I just spoke with the Honda Service Consultant. He said “don’t believe what you read on the internet, these guys are not technicians”. How about that??? Anyway, he clearly believes there is a corelation between mileage and lifetime of the converter. He said many fail by 100k miles and we were very lucky to get 200k miles out of this one. I said I just haven’t read anything that suggests this correlation, that the catalyst doesn’t just get worn out. I said that what is consistent from my reading is that a converter fails for a reason and it’s important to get to the root cause. He said if there was a different root cause than the technician would have found it with his diagnostics. He also said that burning lower quality gasoline (versus high test) allows more detergents and contaminants into the converter and that this will also cause failure. All comments welcome. If his arguments make sense technically, then I am off my bandwagon. Thanks again for your time and willingness to comment. I appreciate the opportunity to learn from your experience and expertise.

A couple of important things. First, its true that lots of internet characters, including myself, are not technicians. But people like me get into doing cars because the technicians are so often wrong. Many of them are just blockheads with computer scanning devices and flow charts, but can’t think their way out of a paper bag. They also don’t drive my car everyday and know it like I do, nor do they give a rat’s tail about whether or not I end up paying for unneeded repairs.

Second, many people on this site are very good technicians and professional mechanics. Tester is one of them. But - and it will likely aggravate him when I say so - his telling of the tale of P04XX codes sounds just like what engineers write up about how things are supposed to work. And things do often work how they are supposed to - but sometimes not. Both that story and the dealer’s story about how the technicians would have found another problem if there was one puts way too much faith in the engine computer knowing what is going on. When I have a problem with a car I normally know it long before the computer decides to count it as a problem. And the computer can’t tell, for instance, whether upstream and downstream O2 sensor reading comparisons are out of spec b/c of the converter or because of something else.

Anyway, the easy point is that if your engine light now stays off then you needed a new catalytic converter. The above notwithstanding, it is the most likely remedy for the P0420.

The comment you got about gasoline from the dealer service rep is wrong if s/he specifically referred to high test. High test is just higher octane meaning it has a higher burning temp. The quality is not about octane (87, 89, 93). If you like you can search on “Top Tier” gasolines and stick to those brands. Also don’t overuse fuel system cleaners or any other kind of gasoline additive, and keep your PCV system clean.

Perhaps most important of all, you don’t need to take this vehicle to a dealer. Find a good reputable, local and independent mechanic. Some will specialize in Hondas. You’ll likely find bills such as this one cut in half. You can also ask them to have one of their techs look it over and get a second opinion as to why the cat failed. They all do eventually.

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You did put quite a few miles on your Honda in the past 6 years.
Just out of curiosity, is it burning any oil, or has the fuel mileage gone down recently?

The most common failure modes for converters is oil or fuel contamination (sloppy engine internals letting in oil, or a fuel injector/spark plug not working correctly). Basically, what happens is that the oil or fuel gets on the converter material, then ignites there, and burns away the material.

Other converters will actually fall apart, which can then have the catalyst material float around. Typically its a white dust. Many a Nissan engine has suffered damage from this material damaging the cylinder walls in the 2.5 4 cylinder engine between 2002 and 2006.

So, if the mechanic looked at the converter, saw that it was in fact coming apart, and used that as the reasoning to replace it, then there’s no reason to question his diagnosis.

Hopefully he replaced the O2 sensors at the same time that he replaced the converter.
If not, call him up, and ask to have them replaced, also.


"He said “don’t believe what you read on the internet, these guys are not technicians”

The “service consultant” is not a technician either, by a long shot.

To have any chance of getting the true story you need to talk directly to the mechanic.

Thanks for asking this - even if it was years ago. Just went through a similar experience with mine 2 weeks after the mechanic had it to repair the heat shields. The maintenance needed indicator was on then & he opted not to check it. Less than a week later, the engine light came on. I read the manual, followed instructions & when the tip was unsuccessful took it in. $1600 later it better not have any additional issues come up next week. Mine has only 106,000 on it & the excuse was it was due to age. Strangely the diagnostic they ran found issues not present at the previous visit where I was told it was in great shape. The diagnostic also missed a long standing issue which I will use when I speak to the manager this afternoon. I too expect to be stuck paying the bill as I need my car running, but advising of dissatisfaction may help with later issues should I return there.

The only mistake made here was taking a 12 year old car with 200K on it back to the dealer for repairs. I suspect an independent shop could have made your CEL go off and stay off for something less than $1900…Much less…

That post is 6+ years old, brought back from the automotive graveyard by the zittfamily !

The car is probably in the crusher, dealership possibly burned down, and the car owner in the witness protection program. :wink:

The best way to tell if the converter alone needed to be replaced AFTER the work was done is to see if you “need” another one (P0420) in a relatively short time.

If yes then you probably didn’t need the first one or the second one or the …

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This is one of the threads with most posted errors that I have seen in a long time. I hope several of you guys have studied cat converters a bit since 2011.

I have followed this topic on here since 1997; and have dealt with it on my 2002 Sienna.

You cannot tell by the code which part is bad. It seems most mediocre mechanics believe if there is a P0420 code that means the cat is bad. Nonsense. All it means is the sensors read similar values going into the cat, and coming out. If a sensor is bad, that will happen with a perfect cat.

Over the years, many of the professional mechanics have said they normally find the sensors are bad and never have to replace the cat unless it is burned up with oil contamination. Others say the 0420 always mean the cat is bad. But, hey, while we are at it, lets replace the sensors, too, and start off with all good stuff. How droll! Note someone said that on this posting. Shame on you.

Nice, only if you are paying the bills.

As one of our best mechanics has said, you need a real mechanic with a graphing scanner to see what is actually wrong. And, then you replace the part which needs to be replaced, period.

If you absolutely cannot find a good mechanic with a graphing scanner, DO NOT REPLACE THE VERY EXPENSIVE CAT Until you have replaced the sensors, which are relatively cheap, and driven it for a while. If a mechanic gives you a bunch of hostility, take your car and run.

Look on this board for Mechanix Files and find a recommended mechanic in your area.

There are two ways sensors fail, unless there are more that don’t know about. First, the filament opens up, or the connections fail. That will show up in the codes as bad sensor.

The other way they can fail is they do not produce the correct signal that correctly represents the actual amount of gases. That may well also show up as a P0420, even if the cat is working fine.

On my 2002 Sienna which had 222,000 miles on it when I was forced to park it by Mexican import laws, I had them all but bad cat. Once the heater opened up, and I drove it back across the US. The only problem was slow warm up of the sensor and on a long trip it was not contaminating. This showed up as a bad sensor in the codes.

Twice, I had P0420 and when the sensors were replaced the cat was okay.

I have never had to replace the cat. The materials in a cat are what is called catalytic, which means they are not changed by normal use, no matter how long. Something has to be wrong to wipe out a cat. It might be an engine running too rich which burns the rare metals. It might be oil or some other sort of contamination.

Let me say categorically: ANY MECHANIC WHO TELLS YOU THAT P0420 ALWAYS MEANS THE CAT MUST BE REPLACED SIMPLY DOES NOT HAVE ANY IDEA WHAT HE IS DOING.Of course, it may mean in a different sense that he knows exactly what he is doing, which is clobbering you for maximum money without a valid reason.

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Did you also have a P01XX code in addition to the P0420?

Shoot it with a .357 a few times and you can be sure it needs replaced…

I tend to disagree with that

I’ve encountered several older vehicles which burn on oil, aren’t running rich, in fact the engine is and always was in perfect mechanical shape. yet the catalytic converter can NOT clean up the emissions enough to pass the smog test

Now let me be more specific . . . these vehicles I’m thinking of don’t have absolutely wasted, plugged up cats, which are literally slowing it down on the freeway

They drive just fine, power is normal, no smell, they are no longer effective at cleaning up the emissions well enough to pass that smog test

the before and after results of the 5 gas analyzer also prove it

But what do I know . . . ?

It may be poor writing on my part, db, but there is nothing in your posting that I disagree with.

My point is, as best as I can say it, P0420 does not necessarily men the cat is bad.

That cartalk wannabe in Houston, Scotty Kilmer, has said if you take off a bad cat and soak it in dish soap overnight, in some cases, a lot of white stuff comes out and then the metal screen is again functional. We have no stats on what percentage that is. But, the photos look as if the white stuff, whatever it is, formed a coating on the wires inside the cat converator, which means the gases do not touch the rare earth wires at all, unitl you soak them out.

Apparently, in really bad conditions, that fine rare earth wire can be burned up, but one assumes the cat would be red hot, though that is only a guess on my part.

Depends. When I had P0420. which for some time was a lot, no, there was no P01xx failure, just the P0420. When I had a burned out filament, yes, I had a specific sensor failure, which is the way it should be.

What happened was we were going from the Snow Zone, though it was summer, back to McAllen. My wife decided she wanted coffee so we pulled off at a big truck stop. The instant I started the car again, the code came on. I had my scanner so knowing I had another 1400 miles to go read the code, and it said bad heater, which I for personal reasons think of as a filament, as on a light bulb, which is very close to what the heater is. And, light bulbs normally burn out on turn-on when the surge current is at its highest. I worked at one time on long-life military light bulbs, so If someone wants to argue, have at it. Our files had electron microscope photos of failed filaments all the way down their life expectancy.

Once I realized it was only the heater, which is used to heat up the sensor fast at start-up, I realized on a long trip like that, it was going to be hot almost all 1400 miles without the heater working and waited until I got back to McAllen to have it replaced. Only twice a day was it started cold, and the amount of smog produced during that minute or two until the exhaust gets hot is trivial.

And, after it was replaced, the intermittent P0420 that had plagued me (intermittently) for quite some time, went away and has never come back, which to me meant the two functions of the sensor both ere involved, the heater had a failure and the sensor function at times was not adequate to produce the difference between the gozinta and the gozouta. Two separate functions, two separate failures on the same part. .

Proving once again that the P0420 on this car at that time was a bad sensor, not a bad cat, which is what I have tried, perhaps poorly, to say.

Even though the heater circuit had failed, and resulted in that code, the sensor itself had probably literally slowed down over the years, but not enough to set a “lazy oxygen sensor” code

Yes, as you essentially said, by replacing the sensor, you killed 2 birds with 1 stone

I don’t know of a way to prove conclusively a cat is bad. With a P0420 showing the shop would normally check the applicable O2 sensors are working, and their outputs are making sense, the air fuel mixture is correct, no lean or rich codes or abnormal fuel trims, the intake manifold vacuum measurement is normal, and that the engine is running properly, has been properly maintained, routine maintenance up to date, and not missing. If all that tests out ok, then the cat most likely needs replacement.