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Car Forum / Ford / Ford Cars / March 2009P0420 and bad oxygen sensors
That's why owners shouldn't replace catalytic converters without checking sensors first: Feb 2009 Motor.com article: http://www.motor.com/magazine/pdfs/022009_02.pdf >That's why owners shouldn't replace catalytic converters without >checking sensors first: > >Feb 2009 Motor.com article: >http://www.motor.com/magazine/pdfs/022009_02.pdf Helps to have a 4 gas analyzer to check the exhaust pre and post cat - then you KNOW if the cat is working. Also having a good scanner that can "track" the O2 sensor helps - you can see what the sensor is doing. Often switching sensors from pre to post will also tell you if the sensor is the problem. If moving the sensors still gives the same code, without an o2 sensor code, it id VERY likely the cat IS week. A catalytic converter is VERY robust, and unless you feed the car leaded gasoline or some other unauthorized substance, the CAT ought to last the life of the car. The only CAT that I have ever replaced was physically damaged by reason of losing a battle with a very large hammer. I had an '89 Ford motorhome (motorhome on a Ford chassis) that I thought had a CAT problem in about 2003. The CAT was fine. The muffler was the problem. The muffler was a significantly less costly repair. The motorhome is still going strong with the same CAT that I thought needed replacing back in '03. I'd not expect any '96 or newer car or truck sold in the USA to have a CAT problem for several more years. The sensors, on the other hand, are not nearly so robust and the environment they live in is very hostile. I'd suspect a sensor failure long before a CAT problem. > That's why owners shouldn't replace catalytic converters without > checking sensors first: > > Feb 2009 Motor.com article: > http://www.motor.com/magazine/pdfs/022009_02.pdf >A catalytic converter is VERY robust, and unless you feed the car leaded >gasoline or some other unauthorized substance, the CAT ought to last the [quoted text clipped - 13 lines] > environment they live in is very hostile. I'd suspect a sensor failure > long before a CAT problem. Pre OBD II vehicles generally did not have a post-cat O2 sensor so there was no on-board way for the vehicle's diagnostic system to evaluate the performance of the cat so you would not get a check engine light related to cat performance. The only way to evaluate cat performance in a pre-OBD II vehicle is with an exhaust gas analyzer (also the best way to evaluate cat performance in an OBD II vehicle). The catalyst in a catalytic converter does not get used up or wear out so it could theoretically last forever. What happens when cats fail is that the catalyst becomes coated with contaminants like particulate (soot) so that the exhaust gas passing over the catalyst bed does not come in contact with the catalyst so no catalytic action occurs, or the converter develops a hole that allows exhaust gas to escape and cool air to enter, or interior corrosion allows the bed holding the catalyst to collapse or come apart, clogging the flow of exhaust. Your motor home probably saw a lot of highway miles, with a nice hot exhaust that minimized buildup of contaminants on the catalyst, prolonging its life.  SignatureRay O (correct punctuation to reply) >>A catalytic converter is VERY robust, and unless you feed the car leaded >>gasoline or some other unauthorized substance, the CAT ought to last the [quoted text clipped - 33 lines] > exhaust that minimized buildup of contaminants on the catalyst, prolonging > its life. And, to clear up an important detail, I thought my CAT was clogged. My fuel mileage went into the dumper and the power fell off even more. I had every indication of a motor that could not breathe, and I suspected the CAT as the problem. It turn out the muffler has dozens of baffles and stuff inside, and something came apart and blocked the passages that the exhaust goes through. The CAT that failed the encounter with a hammer was having a connection problem, the clamps would not hold because there was not enough bite, I sought to enlarge the hole so the pipes would fit together deeper. The plan was a good one, the execution was bad. Oh well ... So, my position is that the CAT is very robust, and not a likely failure item. If the sensor reports a failure of the CAT, my money is on the sensor first, and the CAT second. >And, to clear up an important detail, I thought my CAT was clogged. My fuel >mileage went into the dumper and the power fell off even more. I had every [quoted text clipped - 10 lines] >item. If the sensor reports a failure of the CAT, my money is on the sensor >first, and the CAT second. Okay, we have a problem here. There are basically two kinds of converter problems. First of all we have problems that actually block the exhaust line, which are the kind you're talking about. And those problems, as you note, are pretty rare. But there are other converter failures, where the converter no longer is effective at reducing emissions. If you have an older car and you live in a state that doesn't do emissions testing, you'll never know when these failures happen. And sadly, they happen all the time. If you have a newer car with a second O2 sensor on the output of the converter, and the converter fails, you'll get an error message saying that the converter has failed. And if you are in a state with emissions testing, you'll be told you didn't pass the test. You'll find if you look that the average lifespan of a converter is somewhat short of 100,000 miles, but that they mostly fail in such a way that you aren't noticing. --scott Signature"C'est un Nagra. C'est suisse, et tres, tres precis." >>And, to clear up an important detail, I thought my CAT was clogged. My >>fuel [quoted text clipped - 37 lines] > aren't noticing. > --scott It is my humble opinion that this type of problem -- loss of effectiveness -- is rare. I would suggest that a CAT should last the life of the car for the vast majority of owners. Yes, the CAT might fail, but I suggest that the first thing to fail will be the sensor. The sensor is a relatively fragile device that lives in a hostile environment. The sensor will fail before the CAT. I've not heard of a lifespan for a CAT. I drove a '94 BMW until it read 225,000 miles and was hit by another driver. I have a current '94 BMW with over 130,000 miles and the CAT is strong. My daughter has an '00 BMW that is quickly approaching 150,000 miles and the CAT works good. I have a '97 F150 with 110,000 miles and no problems with the CAT. Every car I own has more miles than you suggested as the life of the CAT. Nobody in my family has ever bought a CAT, and the only CAT I've bought was to replace the one that I badly deformed while trying to correct an unrelated exhaust system problem. I'm not sure there is a stated life for a CAT, and I seem to recall reading that they want the CAT to be a life item for the car that it is installed in. As far as I know, the only thing that can happen, short of physical damage -- the seams opening, that sort of thing -- is that the operator can feed the vehicle's fuel supply some kind of product that is not good. I don't know what not to feed a CAT, other than leaded gasoline, which I don't even know where one could buy that anymore. I suppose there's a possibility that aviation gas might be leaded -- but I don't know one way or the other -- and it could be put into a car. My advice is that if one gets a Failed CAT Code from the OBDII system, test and replace the sensor before suspecting the CAT itself as a failure item. The OBD II system has a sensor that looks specifically at the CAT, but the actual failure item is the sensor itself, and the OBD II system has not got a means of checking and evaluating the sensors themselves. Any code that involves a sensor in the exhaust stream is likely to be caused by the sensor itself, not the hardware that the sensor is looking at. >It is my humble opinion that this type of problem -- loss of >effectiveness -- is rare. I would suggest that a CAT should last the life of >the car for the vast majority of owners. Yes, the CAT might fail, but I >suggest that the first thing to fail will be the sensor. Could be, but how would you know? >I've not heard of a lifespan for a CAT. I drove a '94 BMW until it read >225,000 miles and was hit by another driver. I have a current '94 BMW with >over 130,000 miles and the CAT is strong. My daughter has an '00 BMW that is >quickly approaching 150,000 miles and the CAT works good. I have a '97 F150 >with 110,000 miles and no problems with the CAT. How do you know any of the converters are good? Are you in a place where you are getting annual emissions testing? >Every car I own has more >miles than you suggested as the life of the CAT. Nobody in my family has >ever bought a CAT, and the only CAT I've bought was to replace the one that >I badly deformed while trying to correct an unrelated exhaust system >problem. I'm sure this is the case, but that's not to say you weren't driving the car with a completely ineffective converter for hundreds of thousands of miles. --scott Signature"C'est un Nagra. C'est suisse, et tres, tres precis." >>It is my humble opinion that this type of problem -- loss of >>effectiveness -- is rare. I would suggest that a CAT should last the life [quoted text clipped - 3 lines] > > Could be, but how would you know? Check the sensor. >>I've not heard of a lifespan for a CAT. I drove a '94 BMW until it read >>225,000 miles and was hit by another driver. I have a current '94 BMW with [quoted text clipped - 6 lines] > How do you know any of the converters are good? Are you in a place where > you are getting annual emissions testing? Yes. >>Every car I own has more >>miles than you suggested as the life of the CAT. Nobody in my family has [quoted text clipped - 6 lines] > car with a completely ineffective converter for hundreds of thousands of > miles. Except I would fail smog check long before I drove thousands of miles, not to mention tens, or hundreds of thousands. >>>And, to clear up an important detail, I thought my CAT was clogged. My >>>fuel [quoted text clipped - 74 lines] >involves a sensor in the exhaust stream is likely to be caused by the sensor >itself, not the hardware that the sensor is looking at. I would suggest that you don;t know what you are talking about. The sensor that monitors the catalyst is just a second o2 sensor. If the second sensor tracks the first sensor, the converter is not functioning. It's that simple. It is possible, but not likely, that the conveerter could fail in such a way that the sensor would not pick up the failure, but extremely unlikely that the sensor would indicate a bad cat if it was functioning properly. Both sensors would need to fail, and in such a way that they did not cause an O2 sensor failure code, in order for this to occurr. EXTREMELY unlikely, when you actually know how they work. When I say it is POSSIBLE the cat could fail and not be caught by the sensors, it is because the oxidation catalyst uses up oxygen, and the reduction catalyst frees up oxygen, but generally not the same amount each way. The front O2 sensor oscilates with the O2 concentration of the exhaust gas. When the exhaust is lean, the oxidation catalyst works making co into co2 and burning off unburned hydrocarbons.- so the second sensor does not track. When the exhaust is rich the reduction catalyst works,removing O2 from NOX, making nitrogen and O2. The O2 is generally stored by the cat to feed the oxidation catalyst, so the O2 concentration of the exhaust remains more or less constant at the back sensor. Multiple mode failures in the converter could, concievably, occur which would provide a relatively constant O2 level at the rear sensor without fully treating the HC, CO and NOX. >>>>And, to clear up an important detail, I thought my CAT was clogged. My >>>>fuel [quoted text clipped - 105 lines] > reduction catalyst frees up oxygen, but generally not the same amount > each way. I know what the sensors monitor. What I said was that there is no sensor to monitor the sensors. The sensors send false data, particularly in the case of the O2 sensors _because_ the environment that these sensors operate in is very hostile. Given the specific code being discussed, P0420, all I'm saying is that the sensor is more likely a failure item than the CAT. I'm not looking at a CAT failure that is not reported, I'm looking at a reported CAT failure that does not really exist. We HAVE a code. It specifically points to the after-CAT sensor because the code that is generated can never happen from an upstream sensor. We have a false report of an very costly item failing. That's my story, and I'm sticking to it. ><snipped> I'm not looking at a CAT failure that is not reported, I'm >looking at a reported CAT failure that does not really exist. [quoted text clipped - 4 lines] > We have a false report of an very costly item failing. That's my story, > and I'm sticking to it. We've been through this before. Your premise that the code that is generated can never happen from an upstream sensor is incorrect. P0420 is generated IF the signal from sensor #2 (post-cat) looks like the signal from sensor #1 (upstream sensor). When the system is warmed up, the signal from S1 oscillates, and on an oscilloscope, looks something like a square wave because the oxygen content in the exhaust gas is oscillating. After the exhaust passes through the cat, the O2 content in the gas is evened out so that the signal that S2 puts out has less variance in amplitude (height of the wave) and frequency (number of waves per minute) and so the signal from S2 looks different from the signal from S1. If the cat is bad, or if you took the cat out of the system and replaced it with a piece of pipe, the O2 content doesn't get evened out and so the signal from S2 looks like the signal from S1. IOW, the signal from S2 looks dead if the cat is working and looks lively when the cat is not working. If the ECU sees a lively signal, that means the cat isn't working and it sets P0420. If you get P0420, it is a good idea to check the sensors because it is easy to check, they generally have a shorter life than a cat, a sensor is cheaper than a cat, and a bad S1 can cause premature cat failure, but the cat should also be changed because the likelihood of a false P0420 is almost nil. SignatureRay O (correct punctuation to reply) >> I would suggest that you don;t know what you are talking about. The >> sensor that monitors the catalyst is just a second o2 sensor. If the [quoted text clipped - 19 lines] >specific code being discussed, P0420, all I'm saying is that the sensor is >more likely a failure item than the CAT. With OBD2 the sensors are virtually self checking - There ARE conditions that can slip by, but they are rather rare. On pre-obd2 cars a lazy sensor would not be caught - but the computer knows what a GOOD sensor should do, and if the signal does not behave the way it is expected to, it gets flagged (too few counts, or too narrow a range). >I'm not looking at a CAT failure that is not reported, I'm looking at a >reported CAT failure that does not really exist. Almost unheard of in the real world. There is no documented failure mode of the sensors that could indicate a non-operative cat when in fact it was working. Now, this does NOT mean that the cat might not be functioning due to outside forces - but in that case you WOULD get other codes. The measure of a tech is if he knows which codes are primary, and which are secondary. A good tech can almost "smell" which is the cause, and which is the effect. Testing pinpoint charts cannot do that - it comes from experience and understanding. If you follow the troubleshooting chart in the manual you will likely EVENTUALLY find the problem - but you will go down a lot of dead ends first - and may replace a few parts that are not required. A GOOD tech will start at the other end, and rule out whole blocks of tests at once, without having to replace good parts - and he'll reach retirement age with a full head of hair. >We HAVE a code. It specifically points to the after-CAT sensor because the >code that is generated can never happen from an upstream sensor. > >We have a false report of an very costly item failing. That's my story, and >I'm sticking to it. >>> I would suggest that you don;t know what you are talking about. The >>> sensor that monitors the catalyst is just a second o2 sensor. If the [quoted text clipped - 26 lines] > GOOD sensor should do, and if the signal does not behave the way it is > expected to, it gets flagged (too few counts, or too narrow a range). All of this discussion came about from an article posted that said to not be too quick to replace a CAT due to a P0420 report. Among other causes, the author makes clear that the sensors get lazy. <quote> Lazy oxygen sensors can distort the data and confuse the PCM. </quote That's precisely what I have been saying. Unless one has done something to contaminate a CAT, a lazy sensor is going to report a false condition. In this instance, the after-CAT sensor will report that the CAT is not doing it's job. I'd be looking at the sensor first before I'd be ordering a new CAT. >>>> I would suggest that you don;t know what you are talking about. The >>>> sensor that monitors the catalyst is just a second o2 sensor. If the [quoted text clipped - 43 lines] > report that the CAT is not doing it's job. I'd be looking at the sensor > first before I'd be ordering a new CAT. A lazy O2 sensor will not set P0420. P0420 is set when sensor #2 sees a "lively" signal, indicating that the cat is not working. If the cat is working, the signal from sensor #2 would seem kind of lazy, and the "lazy" signal would indicate a good cat. SignatureRay O (correct punctuation to reply) >>>>> I would suggest that you don;t know what you are talking about. The >>>>> sensor that monitors the catalyst is just a second o2 sensor. If the [quoted text clipped - 47 lines] > working, the signal from sensor #2 would seem kind of lazy, and the "lazy" > signal would indicate a good cat. Different manufactures have different strategies in the pcm to test the cat. If the strategy is to drive the exhaust lean/rich or rich/lean and measure the time delay between the oxygen sensors, then a lazy front sensor could set a p0420/p0430. >>>>>> I would suggest that you don;t know what you are talking about. The >>>>>> sensor that monitors the catalyst is just a second o2 sensor. If the [quoted text clipped - 52 lines] >measure the time delay between the oxygen sensors, then a lazy front >sensor could set a p0420/p0430. A lazy front sensor, yes. Rear sensor, no. And OBD2 standard virtually dictates the front sensor will code before the cat under this condition. The only likely exception would be a vehicle with a linear O2 sensor (LAF) because it doesn't clock the same. It can maintain a constant A/F ratio instead of "averaging" - but it would require a different cat setup. The traditional 3 way cat REQUIRES rich and lean transients in order to function. No lean transitions, and the CO/HC oxidation bed does not work - no rich transitions and the NOX reduction bed does not function. I'm not sure how constant ratio systems handle the catalyst, but I know pre-OBD2 Honda Civic VX vehicles have a BEAR of a time passing E-test in Ontario. They run terribly lean, and generally fail (HC?) dismally. >>>>>> I would suggest that you don;t know what you are talking about. The >>>>>> sensor that monitors the catalyst is just a second o2 sensor. If the [quoted text clipped - 54 lines] > measure the time delay between the oxygen sensors, then a lazy front > sensor could set a p0420/p0430. True, but in that scenario, the codes were not caused by the post-cat sensor. SignatureRay O (correct punctuation to reply) >>>> I would suggest that you don;t know what you are talking about. The >>>> sensor that monitors the catalyst is just a second o2 sensor. If the [quoted text clipped - 42 lines] >report that the CAT is not doing it's job. I'd be looking at the sensor >first before I'd be ordering a new CAT. Actually a lazy after cat sensor would be more likely to pass a bad cat than to condemn a good one. The front sensor clocks, and the rear one doesn't (because it's lazy) gives you a false pass, not a false fail. If the front sensor gets lazy and doesn't clock, OBD2 sets a code. If it narrows it's response, OBD2 sets a code. If it fails biased rich, the vehicle runs too lean and GENERALLY sets a misfire code. It would not set a catalyst code. If it bises lean, the engine runs rich so the oxidation catalyst MIGHT lose efficiency, in which case it COULD set a code. So yes, it is POSSIBLE that a bad sensor could cause a catalyst failure code, but EXTREMELY unlikely - and it would be the PRE cat sensor, not the post. A postcat sensor failure will almost inevitably cause a sensor code, not a cat code. There is virtually no sensor failure mode that would cause a post-cat sensor to clock if the cat is functioning. That said, a perfectly good cat can fail to function under certain conditions - like an exhaust leak ahead of the converter - but that will GENERALLY throw a code for rich limit exceded or something similar. (sensor says lean, engine attempts to richen to correct, and goes beyond a predertermined limit attempting to correct a problem that REALLY does not exist) So yes, while it is POSSIBLE a bad sensor could >>>A catalytic converter is VERY robust, and unless you feed the car leaded >>>gasoline or some other unauthorized substance, the CAT ought to last the [quoted text clipped - 40 lines] > stuff inside, and something came apart and blocked the passages that the > exhaust goes through. Yup, the effect is like Eddie Murphy sticking a bannana up the tailpipe in Beverly Hills Cop. > The CAT that failed the encounter with a hammer was having a connection > problem, the clamps would not hold because there was not enough bite, I [quoted text clipped - 4 lines] > item. If the sensor reports a failure of the CAT, my money is on the > sensor first, and the CAT second. I don't think that cat failure is as rare as you think; it is just that cat failure is not detected very often, which is why the OBD II standard included a way to check cat efficiency. SignatureRay O (correct punctuation to reply) >>>A catalytic converter is VERY robust, and unless you feed the car leaded >>>gasoline or some other unauthorized substance, the CAT ought to last the [quoted text clipped - 48 lines] >item. If the sensor reports a failure of the CAT, my money is on the sensor >first, and the CAT second. You'd be surprised how marginal many cats are from new - the calaysts are expensive so as little as possible active material is used. Anything goes wrong that reduces the effectiveness of the cat AT ALL and you fail your e-test. Coolant leak burning antifreeze can poison the cat. The O2 sensor goes bad, causing the engine to run rich - dead cat. Burning oil? Dead cat. Leaded gas? dead cat. Lots of reasons for cats to fail - and many do. The thing to remember is you need to solve the problem that killed the cat - as they GENERALLY do not die on their own. MANY times a bad O2 sensor will indicate a bad cat - and sometimes it will CAUSE a bad cat. >Lots of reasons for cats to fail - and many do. The thing to remember >is you need to solve the problem that killed the cat - as they >GENERALLY do not die on their own. MANY times a bad O2 sensor will >indicate a bad cat - and sometimes it will CAUSE a bad cat. So how can one remove it permanent and still have the car computer control car correctly? I have 2000 Mazda Protege ES with sensor before and after cat >>Lots of reasons for cats to fail - and many do. The thing to remember >>is you need to solve the problem that killed the cat - as they [quoted text clipped - 7 lines] > I have 2000 Mazda Protege ES with sensor > before and after cat What do you want to remove permanently? SignatureRay O (correct punctuation to reply) >>Lots of reasons for cats to fail - and many do. The thing to remember >>is you need to solve the problem that killed the cat - as they [quoted text clipped - 7 lines] >I have 2000 Mazda Protege ES with sensor >before and after cat Remove what? On a 2000 you need both sensors and the cat. >>I have 2000 Mazda Protege ES with sensor >>before and after cat > Remove what? > On a 2000 you need both sensors and the cat. remove the cat >>>I have 2000 Mazda Protege ES with sensor >>>before and after cat >> Remove what? >> On a 2000 you need both sensors and the cat. > > remove the cat Why do you want to remove the cat? SignatureRay O (correct punctuation to reply) >>>>I have 2000 Mazda Protege ES with sensor >>>>before and after cat [quoted text clipped - 4 lines] > >Why do you want to remove the cat? Cause I don't have the money to replace it On Mar 19, 12:30 pm, m...@privacy.net wrote: > >>>>I have 2000 Mazda Protege ES with sensor > >>>>before and after cat [quoted text clipped - 6 lines] > > Cause I don't have the money to replace it I take no position about replacing or not replacing the bleeping bleeped up cat but this had been a very interesting and informative discussion.... >>>>>I have 2000 Mazda Protege ES with sensor >>>>>before and after cat [quoted text clipped - 6 lines] > > Cause I don't have the money to replace it Removing the cat won't serve a purpose because without the cat, you will have a huge exhaust leak. Even if the cat is not working, you would be better off leaving it in place until you have saved up enough money to replace it. Cheap aftermarket cats start in price from around $75 Of course, you get what you pay for, and those cheap parts will probably only last long enough to pass an emissions test. SignatureRay O (correct punctuation to reply) >>>>>I have 2000 Mazda Protege ES with sensor >>>>>before and after cat [quoted text clipped - 6 lines] > >Cause I don't have the money to replace it And it needs to be replaced, why? It will not effect the running of the car - and removing it cannot be done legally, nor can you fool the control system into thinking it is there if it is removed. Find the money, drive it as is, or park the car. >>>I have 2000 Mazda Protege ES with sensor >>>before and after cat >> Remove what? >> On a 2000 you need both sensors and the cat. > >remove the cat Why? It has no effect on the running of the engine - no effect on power or mileage. (mabee a BIT of exhaust restriction) |
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