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#11
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Request for diagnostic advice to pinpoint least expensivesolution to resolve persistent P0421 OBDII DTC
On 11/17/2020 11:03 PM, Arlen Holder wrote:
> For the record, here's a description of what the computer checks for: > <http://www.smogtechinstitute.com/smogtech/images/PDFs/OBDIIDataInterpretation.pdf> > > 1. The computer looks to see if the heated oxygen sensor voltage rises > above 600mV and falls below 300mV and switches in less than 100ms. > > 2. When the cat is working properly there is almost no switching action > > > Oh Arlen the cat does not work It just sits on my lap all day. But you is so smart anyway. |
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#12
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Request for diagnostic advice to pinpoint least expensive solution to resolve persistent P0421 OBDII DTC
This is a pretty good video (IMHO), on how the rear sensor works:
o <https://youtu.be/afio03Sv_MU> No wonder my sensor readings jump around; it's a narrow band sensor! o Rich will read around 900mV; lean will read around 100mV And it's tested for seven things a. Rich to lean switch time b. Lean to rich switch time c. Frequency of sensor transitions d. Rich to lean sensor threshold voltage e. Lean to rich sensor threshold voltage f. Low sensor voltage for switch time calculation g. High sensor voltage for switch time calculation Under acceleration, the mixture will be rich (reading around 900mV). o When we slow down, the mixture will be lean (reading around 100mV). While idling or coasting, the ECU will constantly change the air:fuel ratio between slightly rich & slightly lean to analyze how the rear sensor responds. The rear sensor shouldn't switch frequently even if the front sensor does, which indicates healthy high oxygen capacity of the catalytic converter. Low switching speed is greater than 24 seconds, whereas high switching speed is less than 1 second (which would be indicating that the catalytic converter has a low oxygen storage capacity). The oxygen sensor needs to be at around 315 degree Celsius to operate correctly, so during a cold start the rear sensor will be at around 300mV flat, but when it heats up (after about 15 to 20 seconds), it will start to switch between lean and rich. While driving at a constant speed (constant load), the rear oxygen sensor should switch slowly (which is a sign of a healthy catalytic converter with high oxygen capacity). When coasting, the rear sensor should drop down to a steady 100mV; when accelerating it should go rich at around 900mV. If you stab the gas pedal four times, you should see four spikes from lean (100mV) to rich (900mV) of the rear oxygen sensor voltage. |
#13
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Request for diagnostic advice to pinpoint least expensivesolution to resolve persistent P0421 OBDII DTC
Arlen Holder wrote:
> The goal is to learn how to diagnose a P0421 using an OBDII scanner. > > Request for diagnostic advice to pinpoint least expensive solution > to resolve persistent P0421 OBDII DTC given... If you have access to a laser temp device or can borrow one... they seem to be everywhere nowadays... if the cat is doing its catty thing then the outlet temp will be a lot higher then the inlet temp. Run it for a 15 sec or more at ~2000 rpm then measure the temps, ~300-400F inlet and 600-800F outlet. If the ratio is lower then it may not be catalyzing very well and will give high HC readings at the tail pipe. In the old days a good working cat would start grass fires. Now they are better shielded so try to get the temp reading of the cat and not the shield. If it were my car I would also take the O2 sensors off and bench test them. |
#14
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Request for diagnostic advice to pinpoint least expensivesolution to resolve persistent P0421 OBDII DTC
On 11/17/2020 11:50 PM, Arlen Holder wrote:
> If you stab the gas pedal four times, you should see four spikes from lean > (100mV) to rich (900mV) of the rear oxygen sensor voltage. > Oh Arlen, you get me so hot when you talk about stabbing four times but you cum in two. |
#15
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Request for diagnostic advice to pinpoint least expensive solution to resolve persistent P0421 OBDII DTC
Arlen Holder > wrote:
>On 17 Nov 2020 23:57:22 -0000, Scott Dorsey wrote: > >> Okay. You're getting an error which is saying that the ratio between the >> forward O2 sensor and the rear O2 sensor is not what it should be. > >I appreciate your advice as I'm not quite sure what readings to look at. >o Let me know what information you need to help me figure out what to do. Read the message over again. What you want to know is in the graphs for the two O2 sensors and how they track. Don't worry so much about the ratio itself, worry about how they track. But you will not figure the problem out just by looking at the plots, you will figure the problem out by thinking about how the emission control system works. >When do I take the readings once the engine is warmed up? All the time. You take them before it warms up, you take them after it warms up, you take them over the course of driving around. What you are looking for is trends and correlation more than specific values. >> The shape of the time-domain curves >> will be different from one model to another, but just watch. The second O2 >> sensor might be bad... the converter might be bad..... and it's -possible- >> that the converter is just overwhelmed with junk (although that it is least >> likely of the three since you aren't seeing any other errors). But watch >> the numbers. >> And you want to look at it for as long as you can... because you can have a >> loose O2 sensor cable, or a bad engine ground, and see weird spikes on those >> graphs that occur very very seldom but which can lead the computer to set >> an error. > >I don't have the necessary equipment to save or see graphs. >o I can only see the live data as it happens. Well, that's useless. So you're going to have to plot it on semilog paper by hand. Your scanner does not provide ANY plotting functions? --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#16
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Request for diagnostic advice to pinpoint least expensivesolution to resolve persistent P0421 OBDII DTC
Paul in Houston TX > wrote:
> >If you have access to a laser temp device or can borrow one... they seem >to be everywhere nowadays... if the cat is doing its catty thing then >the outlet temp will be a lot higher then the inlet temp. Run it for a >15 sec or more at ~2000 rpm then measure the temps, ~300-400F inlet and >600-800F outlet. If the ratio is lower then it may not be catalyzing >very well and will give high HC readings at the tail pipe. In the old >days a good working cat would start grass fires. Now they are better >shielded so try to get the temp reading of the cat and not the shield. And the vehicle is long past the age where it should need a new converter. But that doesn't mean you shouldn't make sure there is no possibility of something else damaging the converter. Because you don't want to swap out the converter and find the problem is fixed but returns. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#17
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Request for diagnostic advice to pinpoint least expensive solution to resolve persistent P0421 OBDII DTC
On 18 Nov 2020 13:20:41 -0000, Scott Dorsey wrote:
> And the vehicle is long past the age where it should need a new converter. > > But that doesn't mean you shouldn't make sure there is no possibility of > something else damaging the converter. Because you don't want to swap out > the converter and find the problem is fixed but returns. Hi Scott, I'm learning a lot, as I didn't even know the voltage range of the lambda sensors, nor what low versus high voltage meant when I opened this thread. I appreciate that, so far, you're the only one who posted who knows anything, which I very much admire and hope to learn what you can impart. As you know, you can't diagnose without understanding, so I've been boning up on how the system works, where I have the same attitude as you do, which is it could be any number of things, but replacing them without first confirming for sure they're definitely bad, is idiotic. A _lot_ of people fix a lot of things that way; but it's still idiotic. o I see now that a _graph_ is almost essential, for proper diagnosis. Given the moving nature of a graph, I learned from this video which shows, in real time, what the voltage graph should look like for the rear sensor: o Understanding & Live Graphing of the Rear O2 Sensor #19, by OBD4Everyone <https://youtu.be/afio03Sv_MU> Given I don't (yet) have a way to graph the results, this video shows how to diagnose using just the live data, which is the redneck way to do it: o P0420 How To Diagnose A Bad Catalytic Converter, by EricTheCarGuy <https://youtu.be/9VZ5K8n5jj0> Speaking of redneck diagnostics, this video shows how a guy taps into the wires, which I think is too invasive given wire coatings, stainless steel wires, etc., but what's interesting is _how_ he forces a "lean" reading and a "rich" reading out of the oxygen sensor to look at fuel trim reactions: o Test O2 Sensor.mov, by realfixesrealfast <https://youtu.be/KxhrCl3B22M> Literally, he grounds the sensor to simulate lean conditions (to watch the fuel trim respond by adding fuel); and he puts himself as the voltage divider resistor to force the oxygen sensor to output rich conditions. While his approach (e.g., feeding propane into a vacuum port to simulate rich conditions) is a bit too much for me, we can learn a lot from redneck methods because they distil the problem down to the essential components. In summary, when I opened this thread, I had no idea what to look for; I still am not sure; but I'm learning to associate low voltage (about ~100mV to about ~300mV) as a lean condition reading - & higher voltage (about ~700mV to ~900mV) as a rich condition - neither of which did I know prior. I'm also slowly starting to understand that the cat acts sort of like a capacitor for oxygen, where the fluctuations of the leading sensor are damped in the readings of the trailing sensor, such that the rear lambda sensor should be steadier at around a stoichiometric ~500mV even as the front lambda sensor wildly fluctuates between lean & rich. If that cat capacitor isn't functioning well, then the voltage fluctuations in the leading sensor will be more closely mirrored in the trailing sensor. A key distinction I need to wrap my head around though, which you seem to have caught upon in the very beginning, is that this is a "warmup" error: o What Does P0421 Mean? <https://www.fixdapp.com/blog/p0421-code> The keyword here is "warmup" (which only you caught in the initial post). "the Bank 1 Catalytic Converter isn't warming up sufficiently" This makes the problem harder to diagnose since, as you seem to have realized instantly, it will only show up during the warmup process. Worse, it happens only "most" of the time, so it's intermittent (i.e., it can go seconds, hours, minutes, or even days between showing up - but it eventually will show up and keep the CEL lit, interestingly, forever). -- Knowledge is just knowing what is while experience is knowing what happens. |
#18
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Request for diagnostic advice to pinpoint least expensivesolution to resolve persistent P0421 OBDII DTC
On 11/18/2020 9:27 AM, Arlen Holder wrote:
> I appreciate that, so far, you're the only one who posted who knows > anything, which I very much admire and hope to learn what you can impart. > That's right Arlen, those other people don't know anything and just waste your time. Only one person knows and gives you a hard on. Just rub it now like you usually do |
#19
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Request for diagnostic advice to pinpoint least expensive solutionto resolve persistent P0421 OBDII DTC
Arlen Holder wrote:
> On 18 Nov 2020 13:20:41 -0000, Scott Dorsey wrote: > >> And the vehicle is long past the age where it should need a new converter. >> >> But that doesn't mean you shouldn't make sure there is no possibility of >> something else damaging the converter. Because you don't want to swap out >> the converter and find the problem is fixed but returns. > > Hi Scott, > > I'm learning a lot, as I didn't even know the voltage range of the lambda > sensors, nor what low versus high voltage meant when I opened this thread. > > I appreciate that, so far, you're the only one who posted who knows > anything, which I very much admire and hope to learn what you can impart. > > As you know, you can't diagnose without understanding, so I've been boning > up on how the system works, where I have the same attitude as you do, which > is it could be any number of things, but replacing them without first > confirming for sure they're definitely bad, is idiotic. > > A _lot_ of people fix a lot of things that way; but it's still idiotic. > o I see now that a _graph_ is almost essential, for proper diagnosis. > > Given the moving nature of a graph, I learned from this video which shows, > in real time, what the voltage graph should look like for the rear sensor: > o Understanding & Live Graphing of the Rear O2 Sensor #19, by OBD4Everyone > <https://youtu.be/afio03Sv_MU> > > Given I don't (yet) have a way to graph the results, this video shows how > to diagnose using just the live data, which is the redneck way to do it: > o P0420 How To Diagnose A Bad Catalytic Converter, by EricTheCarGuy > <https://youtu.be/9VZ5K8n5jj0> > > Speaking of redneck diagnostics, this video shows how a guy taps into the > wires, which I think is too invasive given wire coatings, stainless steel > wires, etc., but what's interesting is _how_ he forces a "lean" reading and > a "rich" reading out of the oxygen sensor to look at fuel trim reactions: > o Test O2 Sensor.mov, by realfixesrealfast > <https://youtu.be/KxhrCl3B22M> > > Literally, he grounds the sensor to simulate lean conditions (to watch the > fuel trim respond by adding fuel); and he puts himself as the voltage > divider resistor to force the oxygen sensor to output rich conditions. > > While his approach (e.g., feeding propane into a vacuum port to simulate > rich conditions) is a bit too much for me, we can learn a lot from redneck > methods because they distil the problem down to the essential components. That is the factory method for testing, not "redneck" as you seem to think. As for being invasive, not even close as long as you seal the holes you make or use a backprobe method. > > In summary, when I opened this thread, I had no idea what to look for; I > still am not sure; but I'm learning to associate low voltage (about ~100mV > to about ~300mV) as a lean condition reading - & higher voltage (about > ~700mV to ~900mV) as a rich condition - neither of which did I know prior. > > I'm also slowly starting to understand that the cat acts sort of like a > capacitor for oxygen, where the fluctuations of the leading sensor are > damped in the readings of the trailing sensor, such that the rear lambda > sensor should be steadier at around a stoichiometric ~500mV even as the > front lambda sensor wildly fluctuates between lean & rich. > > If that cat capacitor isn't functioning well, then the voltage fluctuations > in the leading sensor will be more closely mirrored in the trailing sensor. > > A key distinction I need to wrap my head around though, which you seem to > have caught upon in the very beginning, is that this is a "warmup" error: > o What Does P0421 Mean? > <https://www.fixdapp.com/blog/p0421-code> > The keyword here is "warmup" (which only you caught in the initial post). > "the Bank 1 Catalytic Converter isn't warming up sufficiently" > > This makes the problem harder to diagnose since, as you seem to have > realized instantly, it will only show up during the warmup process. In simple terms it means that the converter isn't starting to operate as fast as the table in the ECM thinks it should given the engines run time, fuel and air states. The ECM looks at the time the engine started, it's loading, coolant temp, air temp, rpm timing, injector flow, air flow and a couple other sensors and using the tables programmed in it has decided the converter isn't starting to do it's job fast enough based on the numbers they used in the programming. Sometimes this can simply be a bar sensor, IE the rear sensor is sluggish due to contamination or age and the ECM thinks the cat is the issue when it's the sensor. That is the whole point of the graph, you want to see how long it takes on a cold start for the sensors to start working and what the voltage numbers are. The direct testing at the sensor is also the best testing because everything that comes through a scan tool is processed data, it has been known to be wrong multiple times, while direct testing shows the raw data. > > Worse, it happens only "most" of the time, so it's intermittent (i.e., it > can go seconds, hours, minutes, or even days between showing up - but it > eventually will show up and keep the CEL lit, interestingly, forever). -- Steve W. |
#20
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Request for diagnostic advice to pinpoint least expensive solution to resolve persistent P0421 OBDII DTC
On Wed, 18 Nov 2020 11:38:43 -0500, Steve W. wrote:
>> The keyword here is "warmup" (which only you caught in the initial post). >> "the Bank 1 Catalytic Converter isn't warming up sufficiently" >> >> This makes the problem harder to diagnose since, as you seem to have >> realized instantly, it will only show up during the warmup process. > > In simple terms it means that the converter isn't starting to operate as > fast as the table in the ECM thinks it should given the engines run > time, fuel and air states. Hi Steve W., Thanks for your advice, as cat "warmup" is the critical parameter for sure! I agree that the P0421 indicates a catalytic "warmup" fault, where it must be close since I've seen three situations occur multiple times so far: o In most cases, the P0421 is set the instant readiness monitors are full o In other cases, the P0421 is set days after readiness monitors are full o Oddly, once set, it doesn't ever reset (e.g., after 3 good drive cycles) > The ECM looks at the time the engine started, it's loading, coolant > temp, air temp, rpm timing, injector flow, air flow and a couple other > sensors and using the tables programmed in it has decided the converter > isn't starting to do it's job fast enough based on the numbers they used > in the programming. Of course. o The difficult question to answer is what to test to debug during warmup. The lambda sensor is heated, so it only takes ~20 seconds to warm up. o But what's expected warmup time for a cat (& how exactly is it measured?) > Sometimes this can simply be a bar sensor, IE the rear sensor is > sluggish due to contamination or age and the ECM thinks the cat is the > issue when it's the sensor. That is the whole point of the graph, you > want to see how long it takes on a cold start for the sensors to start > working and what the voltage numbers are. Yes. Indeed. I certainly didn't realize this when I originally opened this thread, but I do realize that measuring how long it takes to get to steady state after an overnight rest & morning cold start seems to be a critical diagnostic. > The direct testing at the > sensor is also the best testing because everything that comes through a > scan tool is processed data, it has been known to be wrong multiple > times, while direct testing shows the raw data. Understood. I'm very familiar with flying leads. o I simply prefer not to do that to other people's cars if I don't have to. What I need at the moment is to borrow the vehicle and see what I can determine in the first crucial minute. o I know it only takes about ~20 seconds for the sensors to warm up But I'm not yet sure how to test the time it takes for the cat to warm up. o Especially when it appears to be a borderline condition on this vehicle. -- Diagnostics are useless if we don't first understand how the system works. |
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