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P0401 Question

Discussion in 'Gen 3 Prius Technical Discussion' started by donzoh1, Feb 21, 2024.

  1. donzoh1

    donzoh1 Active Member

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    I correct on assuming that a bad DPFE sensor or circuit problem is the likely cause of P0401 code when no misfire can be heard and there are no misfire codes stored or pending? After the code is cleared, it returns immediately, even when the ICE is not running.
     
  2. Mendel Leisk

    Mendel Leisk Senior Member

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    This is for a gen 3, not the '04? AFAIK it points to insufficient EGR flow. What's the miles, and has EGR been cleaned?
     
  3. donzoh1

    donzoh1 Active Member

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    Gen 3 2013. EGR cooler was cleaned along with manifold. Miles are 225K. My thinking is that misfire codes would eventually be added if there was a chronic flow problem.
     
  4. Mendel Leisk

    Mendel Leisk Senior Member

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    How many miles back was the EGR cleaning? Intake manifold done as well?

    If all that is good, maybe the EGR valve crapping out, what @ChapmanF calls the "ski jump" getting chewed up, valve not opening properly?

    Misfire codes manifest with coolant in combustion chamber?
     
  5. donzoh1

    donzoh1 Active Member

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    Coolant level is steady. Ski Jump was somewhat worn so could be an issue. EGR and manifold thoroughly cleaned in last two weeks with completion of head gasket repair. (Mileage is up from 42 to 47.) Main question is what problems are likely to cause immediate code after clearing. I think a sensor or circuit problem is the likely culprit there.
     
  6. ChapmanF

    ChapmanF Senior Member

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    P0401 is an EGR flow code, nothing to do with misfires or DPFE (on a Prius?!) or anything.

    The report that it comes back immediately after clearing is also downright weird. The code is set according to the result of an EGR flow self test. The only time the flow self test can run is when the engine is warm and you are decelerating with the engine running but the fuel injection cut off. After a genuine code clear, there's not supposed to be any way for that code to come back until the next time that test can run, which requires some actual driving of the car.

    So something in the story isn't adding up.

    It would be interesting to use a scan tool to look at the monitor status flags right after doing that code clear. And also to look at the most recent EGR flow test result.
     
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  7. Mendel Leisk

    Mendel Leisk Senior Member

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    This is news.

    Maybe should have just continued in this thread:

    Head Gasket Failure | PriusChat
     
  8. donzoh1

    donzoh1 Active Member

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    So I'm not hearing or feeling any ICE problems. As long as the only code is P0401, is it likely to cause damage? Would a P0300, 01, etc. come before engine damage? My scanner is kinda cheap but it has read and cleared codes before. I didn't disconnect the 12v tho.
     
  9. ChapmanF

    ChapmanF Senior Member

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    There is a school of thought here on PriusChat that a reduction in EGR flow (which is what a P0401 code tells you about) may be a factor in head gasket damage. Misfire codes may eventually alert you to a damaged head gasket, but only too late.

    I am not one of the strong adherents to that school of thought, but you can definitely read about it here.
     
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  10. donzoh1

    donzoh1 Active Member

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    I did see some serious cylinder wear while replacing the head gasket so I'm probably just going with a JDM motor if this one fails again. is there any way to distinguish the cause of P0401 by sticky plunger, ski jump wear, circuit wiring fault, bad sensor, bad EGR valve, or EGR/intake manifold clog? I just finished a thorough cleaning to probably not a clog.
     
  11. Mendel Leisk

    Mendel Leisk Senior Member

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    Intake manifold passages, because they tend to clog sequentially, I'd put lower on the list of suspects. If you're confident recent cleaning was decent, then I'd suspect EGR valve function first.

    There might be something useful in attached.
     

    Attached Files:

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  12. ChapmanF

    ChapmanF Senior Member

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    Post #6 suggested getting some data, starting with the actual test result of the car's last EGR flow test (it takes the form of a pressure figure, such as in kilopascals, which is really the difference between two MAP readings, one with the EGR valve closed and one with it a certain amount open). That's the easiest thing to start with, and it's also possible to more directly manipulate the valve opening step and observe the raw MAP readings.

    I'm not easily thinking of any promising diagnostic approach that wouldn't start with getting some of that info.
     
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  13. donzoh1

    donzoh1 Active Member

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    Thanks for the repair manual info. I am leaning toward EGR valve as well, in part because of ski jump degradation. I poked around a bit after reading repair manual and found a couple possible vacuum leaks. Disconnected 12V battery and check engine light went away. But, the P0401 is still pending so either the problem persists or it needs a couple of drive cycles?
     
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  14. donzoh1

    donzoh1 Active Member

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    My scanner has live data but not tech stream so I'm doubting that EGR data would be found without tech stream?
     
  15. ChapmanF

    ChapmanF Senior Member

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    "Mode 6" monitor results are a standard OBD-II thing that lots of scan tools can do. The thread linked back in post #6 has examples with people not using Techstream.

    Active tests are less widely supported.
     
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  16. donzoh1

    donzoh1 Active Member

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    More on the P0401...

    I replaced the EGR (valve section only) with a new off brand one (other EGR sections and manifold ports have been recently cleaned). The P0401 code has returned but now says EGR Problem rather than Insufficient EGR Flow. One potential issue could be the MAP Sensor as I broke one of the wires and repaired that during reassembly. MAP reading at freeway speed was around 27 and when decelerating (ICE not running) the reading was around 7.
    IMG_20240228_103639925_HDR.jpg IMG_20240228_103639925_HDR.jpg IMG_20240228_103406925_HDR.jpg IMG_20240228_103639925_HDR.jpg IMG_20240228_103406925_HDR.jpg IMG_20240228_103344504_HDR.jpg
     
  17. donzoh1

    donzoh1 Active Member

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    Tried three photos but not sure I did that correctly.
     
  18. ChapmanF

    ChapmanF Senior Member

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    Is that on a different scan tool? The code itself is the only thing the car says. Whatever fortune cookie a scan tool shows you about what the code "means" is just whatever's baked into the tool.

    For the record, because P0401 is a code in the P0 range that's standardized by SAE, there is an SAE-standardized fortune cookie for it, which I think is "Exhaust Gas Recirculation 'A' Flow Insufficient Detected".
     
  19. donzoh1

    donzoh1 Active Member

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    I think it's Innova 3320RS or something. But, it is still the same P0 code. Given that it didn't change from P0401 by replacing the EGR Valve, is a MAP sensor a likely suspect? This sensor was soaked when I pulled it (before EGR cleaning and intake manifold oil removal). The broken head gasket (now replaced) was causing combustion gases to enter the cooling system. I did break one MAP Sensor wire but repaired that. Does a value of 27hg from that sensor at highway speed and 7hg while coasting (ICE off) seem correct? I'm thinking I should check for sensor oil soaking again?
     
  20. ChapmanF

    ChapmanF Senior Member

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    I kinda suspect they all get wet with oil and it doesn't impair their pressure readings.

    Those readings don't seem unreasonable to me. Is it close to your nearest weather station's barometer when the engine is off? Is it down somewhere around 4 psi (or 8 inHg) when idling with no EGR?

    Are you sure your Innova can't give you Mode 6 results? Innova has a web page about them. Maybe you could check with them about whether your Innova model can show them, and how.

    What is Mode 6? – Innova