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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... 1. College kid's early 2000s Mitsubishi fails OBD-only smog 2. Neighbor asks me to help out (as if I know anything, which I don't) 3. However, I can borrow the vehicle & run live tests via OBDII scanner Question is, what do I look for in terms of "live data" to help diagnose? DETAILS FOLLOW: o 180K miles, circa 2003 or 2004 model year (I think) o Service Engine Soon has been reported to be on for a year o New pre cat 02 sensor had reportedly already been installed 3 months ago o Lambda O2 Sensor always tests OK on the OBDII scanner o CAT always tests OK on the OBDII scanner o All other registers always test OK on the OBDII scanner o The P0421 DTC is set "sometimes" when the CAT register is set o I cleared the DTCs a few times but the P0421 came back each time o I/M Readiness lasts only about a day or three before DTC occurs again o Always the same DTC and always only that single DTC o Warm Up Catalyst Efficiency Below Threshold Bank 1 The question isn't "Does it require a new cat", nor is the question "What parts can we blindly throw at it until it finally works". The question is what "live data" do we look for to _diagnose_ what the problem actually is (and what's the options for repair). My main dilemma that I'm asking for your help on is what do I look for in the "live data" when I take it for a test drive with the OBD scanner? BTW, I'm familiar with the concept of the Federal Test Procedure (also known as the Drive Cycle), but I don't have the specific drive cycle for this specific vehicle (but I can guess based on the drive cycle for other vehicles, although not the same marquee). PHOTOS FOLLOW: o <https://i.postimg.cc/KzTmdCBx/obd01.jpg> Failed smog o <https://i.postimg.cc/nrYpL73W/obd02.jpg> 180,000 miles o <https://i.postimg.cc/65tBmj3x/obd03.jpg> 1 DTC with all registers set o <https://i.postimg.cc/fRbDwVmH/obd04.jpg> P0421 below threshold, bank 1 o <https://i.postimg.cc/437G9j8V/obd05.jpg> Lambda sensor OK o <https://i.postimg.cc/kgh9D7y7/obd06.jpg> CAT OK o <https://i.postimg.cc/901KSB8c/obd08.jpg> EVAP, MIS, FUEL, OK o <https://i.postimg.cc/26nfHhN5/obd07.jpg> Live data page 1 o <https://i.postimg.cc/v8vJcQQC/obd09.jpg> Live data page 2 o <https://i.postimg.cc/vHqCt3xz/obd10.jpg> Live data page 3 o <https://i.postimg.cc/NjKSJm1w/obd11.jpg> Live data page 4 o <https://i.postimg.cc/T3BSZ195/obd12.jpg> What values do I look for? QUESTION: o What do I look for in terms of "live data" to help diagnose? -- The goal is to learn how to diagnose a P0421 using an OBDII scanner. |
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Arlen Holder > wrote:
>The goal is to learn how to diagnose a P0421 using an OBDII scanner. 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. And you are -not- getting an error that is saying that the forward O2 sensor is out of range, which is also important. The fact that you are -not- getting that error tells you that the exhaust coming out of the engine isn't THAT far off. Now, it could still be off. It's still worth doing all the basic sanity checks: change the plugs, make sure the cables are good and there are no signs of arcing, make sure fuel pressure is good and that the exhaust smells okay. The idle RPM should be right on the mark and should not be drifting. If you're spitting unburnt fuel down the tailpipe, the converter is going to be unhappy. Now, look at the data for the two O2 sensors and look at what happens when the engine starts up. The first sensor should climb up into the normal range once it warms up. The second sensor isn't going to get into the normal range until the converter warms up. 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. If this engine has two banks, you can compare the two and that is very very useful information. Since you don't give the engine or model it's hard to tell if that will help you or not though. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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On 11/17/2020 6:06 PM, 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... > > 1. College kid's early 2000s Mitsubishi fails OBD-only smog > 2. Neighbor asks me to help out (as if I know anything, which I don't) > 3. However, I can borrow the vehicle & run live tests via OBDII scanner > > Question is, what do I look for in terms of "live data" to help diagnose? > > DETAILS FOLLOW: > o 180K miles, circa 2003 or 2004 model year (I think) > o Service Engine Soon has been reported to be on for a year This is easy to diagnose. You just sell that POS and get a newer one. You know better, Arlen dear. Dumb kid won't graduate anyway. That is why I married you. |
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In alt.home.repair, on Tue, 17 Nov 2020 23:06:39 -0000 (UTC), Arlen
Holder > wrote: >The goal is to learn how to diagnose a P0421 using an OBDII scanner. Does your scanner give real time data? Most scanners, all cheaper ones, only give the scan code, which you have. Look it up online, see what it means, and see what remedies are suggested for it. That will probably go faster than using the scanner even if it does give real-time data. |
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In alt.home.repair, on Tue, 17 Nov 2020 23:06:39 -0000 (UTC), Arlen
Holder > wrote: >The goal is to learn how to diagnose a P0421 using an OBDII scanner. Diagnostic trouble code (DTC) P0421 stands for “Warm Up Catalyst Efficiency Below Threshold (Bank 1).” It means that the oxygen sensor downstream of your vehicle's catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should. https://www.carparts.com/blog/p0421-...eshold-bank-1/ IMPORTAnt: Although the trouble code P0421 is related to your vehicle’s catalytic converter, it does not always mean that the converter is faulty. It could also indicate other issues, such as a circuit problem or a bad oxygen sensor. And the cat is expensivie to replace, especially for nothing. Here are some of the common triggers of the error code P0421: Faulty Bank 1 catalytic converter Faulty oxygen sensor Issues with the oxygen sensor circuit, such as damaged wires or poor connections Exhaust leak An issue with the PCM, such as software in need of an update |
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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. Hi Scott, 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. OK. That's a good start as I was unaware that was what it tells us. o The main problem is what should that front-to-back ratio be? And under what conditions? > And you are -not- getting an error that is saying that the forward O2 > sensor is out of range, which is also important. It's a 3-month old forward sensor. o I have no idea about the age of the rear sensor though. In the daylight, if it's dry outside, I will ask to look under the vehicle. o There is only one bank (AFAIK). > The fact that you are > -not- getting that error tells you that the exhaust coming out of the > engine isn't THAT far off. From what I've gathered, I agree. It goes hours, sometimes days between the registers being set and the DTC. However I watched the dash light go on the instant the CAT register set. So it's "very" close, I think. > Now, it could still be off. It's still worth doing all the basic sanity > checks: change the plugs, make sure the cables are good and there are no > signs of arcing, make sure fuel pressure is good and that the exhaust smells > okay. The idle RPM should be right on the mark and should not be drifting. Note there is no evidence whatsoever of a misfire. Nor of rough idle. No smoke. No smells. But it's still good to replace the plugs (but that's throwing parts at it). > If you're spitting unburnt fuel down the tailpipe, the converter is going > to be unhappy. Yup. The goal, always, is to not destroy the cat. > Now, look at the data for the two O2 sensors and look at what happens when > the engine starts up. The first sensor should climb up into the normal range > once it warms up. The second sensor isn't going to get into the normal > range until the converter warms up. OK. Thanks. I'll look for that first. a. Start cold (overnight) b. Start & idle & watch the two O2 sensors (pre cat and post cat) c. I'm looking for the pre-cat to get into range sooner than the post cat Are these the two voltage readings for the two oxygen sensors? o O2SLOC B1S12--B2S---- o O2B1S1(V) 0.120 o O2B1S2(V) 0.140 <https://i.postimg.cc/NjKSJm1w/obd11.jpg> When do I take the readings once the engine is warmed up? Here's a reading, for example, taken at a steady 50mph: o O2SLOC B1S12--B2S---- o O2B1S1(V) 0.400 o O2B1S2(V) 0.160 <https://i.postimg.cc/T3BSZ195/obd12.jpg> > 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. Here is a live shot of the data at a steady 60 mph: o O2B1S1(V) 0.120 o O2B1S2(V) 0.180 <https://i.postimg.cc/qRLqQZVv/obd13.jpg> And another stead 60 mph reading: o O2B1S1(V) 0.740 o O2B1S2(V) 0.640 <https://i.postimg.cc/Gtm2fJdJ/obd14.jpg> I'm not sure the speed for this one: o O2B1S1(V) 0.460 o O2B1S2(V) 0.120 <https://i.postimg.cc/YqV9XWVY/obd15.jpg> Here's another reading, at a steady 20 mph: o O2B1S1(V) 0.700 o O2B1S2(V) 0.400 <https://i.postimg.cc/x1zdBdD7/obd16.jpg> And another at an unrecorded speed: o O2B1S1(V) 0.820 o O2B1S2(V) 0.600 <https://i.postimg.cc/3wQwhH3w/obd17.jpg> And another at an unrecorded speed: o O2B1S1(V) 0.500 o O2B1S2(V) 0.800 <https://i.postimg.cc/yNh8YrNy/obd18.jpg> Here's one taken at 50 mph: o O2B1S1(V) 0.140 o O2B1S2(V) 0.760 <https://i.postimg.cc/k4nXVF63/obd19.jpg> Another, albeit in a sharp turn, at 50 mph: o O2B1S1(V) 0.800 o O2B1S2(V) 0.780 <ttps://i.postimg.cc/nz0VqjBs/obd20.jpg> Another, on the highway but at an unrecorded speed: o O2B1S1(V) 0.780 o O2B1S2(V) 0.140 <https://i.postimg.cc/RhYMbkP4/obd21.jpg> Taken shortly thereafter at 55 mph: o O2B1S1(V) 0.820 o O2B1S2(V) 0.700 <https://i.postimg.cc/bYVNJnqW/obd22.jpg> Another on the highway at an unrecorded speed (2500 RPM though): o O2B1S1(V) 0.140 o O2B1S2(V) 0.220 <https://i.postimg.cc/kXVMt544/obd23.jpg> Another on the highway at 60 mph: o O2B1S1(V) 0.400 o O2B1S2(V) 0.160 <https://i.postimg.cc/J49r3WDQ/obd24.jpg> Another on the highway at 60 mph moments later: o O2B1S1(V) 0.620 o O2B1S2(V) 0.300 <https://i.postimg.cc/GmGcqSvn/obd25.jpg> Another on the highway at 60 mph: o O2B1S1(V) 0.120 o O2B1S2(V) 0.180 <https://i.postimg.cc/rp9qL0hh/obd26.jpg> Another on the highway at 60 mph: o O2B1S1(V) 0.440 o O2B1S2(V) 0.240 <https://i.postimg.cc/sXDsfjq3/obd27.jpg> This one at 1,000 rpm (probably idling): o O2B1S1(V) 0.220 o O2B1S2(V) 0.840 <https://i.postimg.cc/fL2ZV4Qm/obd28.jpg> Still at 1000 rpm shortly thereafter: o O2B1S1(V) 0.060 o O2B1S2(V) 0.760 <https://i.postimg.cc/mZnBRGwG/obd29.jpg> Another at 1000 rpm: o O2B1S1(V) 0.600 o O2B1S2(V) 0.480 <https://i.postimg.cc/43XTw4Jp/obd30.jpg> Yet another at 1000 rpm: o O2B1S1(V) 0.240 o O2B1S2(V) 0.140 <https://i.postimg.cc/tTTGQrnT/obd31.jpg> Here's one at 10mph and 2300 rpm: o O2B1S1(V) 0.820 o O2B1S2(V) 0.780 <https://i.postimg.cc/QCKLVHbZ/obd32.jpg> I'm not sure if it's normal for these readings. o I do wish I had a graph - but I only seem to have live data. > If this engine has two banks, you can compare the two and that is very very > useful information. Since you don't give the engine or model it's hard to > tell if that will help you or not though. Only one bank, thank God. ![]() BTW, I just checked. It's a 2004 Mitsubishi Lancer ES, 2.0liter SOHC 4G94 engine -- I appreciate your advice as I'm not quite sure what readings to look at. |
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[snip]
Given that you're in a State that mandates emission inspections... Step 1: use your scanner to _clear_ the error code. If you're very lucky, it won't show back up. Alternatively, if it shows up again every 500 miles, just live with it [a]. If it comes right back after 30 nominal miles (that is, once the system decides to do the rechecks), then you've got to figure out what's wrong In my case, with a different car and a slighrly different OBD code, it turned out (we discovered after _lots_ of misdirection) that the EGR was pouring plenty of garbage into the exhaust, coating the catalytic converter, and messing things up. Once we replaced the EGR this went from getting the error light/message 20 miles after reset to.. the aforemention very roughly every 500 miles. Presumption was/is that there's caked on gunk which is interfering with the converter, but... it's not enough to set it off right away. And, alas in my case, no easy way to pull the converter and clean it. (That's an option with some other cars). [a] So what I did was check the OBD readings fifty miles before my last inspection [b], make sure it was ok, head to the inspection station, stop outside, double check the reading, and bring it in. [b] the OBD ssytem needs to monitor the car for thirty or so miles after a reset, so I couldn't just clear it at the curb. For good measure, which might or might not have made a difference but which made me feel better, I 1: filled the last couple of tanks with ethanol_free gasoline (which was also higher octane) and 2: poured in Luke Oil cleanser into the tank. good luck -- __________________________________________________ ___ Knowledge may be power, but communications is the key [to foil spammers, my address has been double rot-13 encoded] |
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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 from the rear oxygen sensor. 3. To test the rear oxygen sensor, the computer forces a rich or lean mixture that the cat can't compensate for, and then monitors the oxygen sensor voltage. a. Rich to lean threshold voltage b. Lean to rich threshold voltage c. Low sensor voltage for switch time calculation d. High sensor voltage for switch time calculation e. Rich to lean sensor switch time f. Lean to rich sensor switch time g. Minimum sensor voltage for test cycle h. Maximum sensor voltage for test cycle i. Time between sensor transitions |
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According to this discussion on the oxygen sensor reading:
<https://ricksfreeautorepairadvice.com/how-to-read-live-data-from-obd-ii/> 1. At the 14:1 stoichiometric ratio we should get a reading of 450mV, but the pre-cat oxygen sensor will fluctuate around 500mV in use. 2. The post-cat oxygen sensor should be more steady at around 500mV since it's reading "cleaned up" exhaust. 3. However, if the pre-cat sensor is receiving too rich a fuel mixture, it will read a fairly steady 1.0V while too lean will read near 0V. -- According to Autozone for a 2004 Mitsubishi Lancer ES 2.0L FI SOHC 4cyl o Upstream Bosch Part #15514, Walker 350-34173, Denso 234-4739, NTK 21512 o Downstream Bosch Part #15360, Walker 350-34644, Denso 234-4642, NTK 23513 |
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This explains the pressure waves:
<https://www.knowyourparts.com/technical-resources/electrical/measuring-o2-sensor-signals/> 1. The oxygen sensor monitors the content of the pressure waves as they pass the sensor. With each pressure wave the sensor sends a rich or lean mixture voltage to the engine controller. 2. At temperature, the sensor changes resistance as the air/fuel ratio changes. But instead of a gradual change, it switches very quickly from low resistance (less than 10 K ohms when the mixture is rich), to more than 20 K (ohms when the mixture is lean). 3. The engine controller supplies a base reference voltage of one volt to the sensor. 4. Lambda1 is the reference voltage of 450 mV, which represents an air-fuel ratio of 14.7:1 or stoichiometric. 5. The sensor is capable of generating a one-volt signal when there is no oxygen in the exhaust stream. |
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