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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 think you mentioned this but in the test for insufficient EGR Flow, the computer is using MAP Sensor data which means a bad sensor can trigger the P0401.
     
  2. donzoh1

    donzoh1 Active Member

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    I will check on the Innova page.
     
  3. ChapmanF

    ChapmanF Senior Member

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    I don't mean this personally at all, but that's an example of the kind of partial-story reasoning you sometimes see when a person has learned one or two things about how something works but hasn't learned the rest.

    For example, the EGR flow monitor test (which is how a P0401 code gets set) is made by taking two MAP sensor readings, under different conditions, and subtracting them. So there are lots of ways a MAP sensor could be bad without triggering the code. (Say, if the sensor just always reads so many inHg too high or too low; when one such reading is subtracted from another, the errors cancel and the test result won't be affected.)

    Also, the ECM checks for MAP sensor problems (the P0106, P0107, and P0108 codes are for those).

    So for a MAP sensor problem to be the cause of a P0401 (but without you seeing also a P0106, P0107, or P0108), it would have to be (a) a MAP sensor problem sneaky enough not to trigger those other codes, and (b) one that is nonlinear enough to not simply be cancelled when the two MAP readings are subtracted.

    Which is not to say that a MAP sensor problem can't be the cause, but only to say to approach the diagnosis carefully and get as much data as you can.
     
  4. donzoh1

    donzoh1 Active Member

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    Agreed and I'm certainly not an expert here. It's not personal...it's just a fact. My Innova 3100RS doesn't have Mode 6 capability according to their tech support people. It does have Live Data and it can command EGR so I was thinking I could monitor the MAP values while commanding the EGR Valve. I'd need to know what the P0401 parameters are in case it's just a marginal problem. None of the other MAP codes are coming up. I think my wiring repair was correct since it had a P0108 when the wire was broken.
     
  5. donzoh1

    donzoh1 Active Member

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    I did ask their tech support and the guy said it can't but their website says some of their newer ones can.
     
  6. ChapmanF

    ChapmanF Senior Member

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    I'm not sure anybody knows all of them. We know the test runs when the engine is warmed up and you happen to be decelerating from above 25 mph with the engine spinning and the fuel cut off. Then the ECM notes the MAP reading with the EGR valve closed, then opens the valve some, notes the MAP reading then, and subtracts the valve-closed reading. The difference is what's reported as the mode 6 test result.

    We know a difference of around 21 kPa (or 6 inHg) is really good, as in a freshly cleaned system, and the trigger for failing the test is much lower (roughly 1 kPa). But we don't know exactly how far the test opens the valve, or whether it's always the same amount or depends on something else, etc. it would probably be pretty hard to effectively duplicate the test by issuing active-test commands in real time during a few seconds of deceleration. You might get a result but who knows how to compare it to a result from the built-in test?
     
  7. donzoh1

    donzoh1 Active Member

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    Thanks for the info. So I'm guessing either a Mode 6 compatible scanner or tech stream at the things that would work here, other than just plugging in a new sensor. Incidentally, the car does sometimes have a round idle or shudders sometimes while driving, but no other codes such as misfire or specific MAP codes.
     
  8. Brian1954

    Brian1954 Active Member

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    The picture of the scanner shows the P0401 as permanent, which means it is not current. The permanent code will eventually clear, so it looks like you may have fixed the problem by installing the new EGR valve.

    What are the other two trouble codes that the scanner detected?
     
  9. ChapmanF

    ChapmanF Senior Member

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    ^^^ Good catch! I looked right at that picture and didn't spot that. :oops:

    Yes, the permanent code will go away by itself once three consecutive drive cycles have included that self-test and it passed each time, or will go away when you drive the P0401 confirmation pattern once and it passes, if you don't want to wait.
     
  10. donzoh1

    donzoh1 Active Member

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    Thanks. I did clear codes but I guess that permanent code is still stored until completing the drive test cycles. Is the slight shuddering feeling caused by a test?
     
  11. ChapmanF

    ChapmanF Senior Member

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    Shouldn't be related to the test. The test just gets snuck in now and then during deceleration when the engine isn't doing any work.
     
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  12. donzoh1

    donzoh1 Active Member

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    Thanks. Yesterday, I noticed the EV dash lettering came up on deceleration briefly a couple times. I was in ECO mode at the time as usual. The scanner refers to Emissions Related Codes but doesn't have the P0401 stored. Would Torque Pro have access to those?
     
  13. donzoh1

    donzoh1 Active Member

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    P0401 continues either as permanent or as 3 codes...all P0401...either permanent, stored, or pending. Also, rough running at idle or some low power demand situations. I'm thinking either a vacuum leak or bargain basement EGR valve could be culprits here. Would P0456 or something else be set with any vacuum leak? Haven't seen that code.

    I don't know the exact function modes of the EGR valve but given it has six pins, I'm guessing one is a ground and the other five are some level or levels of positive voltage. I might like to bench test if there's a way to get the known good values for different plunger positions.
     
  14. ChapmanF

    ChapmanF Senior Member

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    As covered in a bunch of older threads, it's just a six-terminal unipolar stepper motor.

    [​IMG]

    There's nothing about it that indicates how far open it is. When the car doesn't want EGR at all, it pulses the coils in the right sequence to step the valve toward closed. The valve closes as far as it can and, naturally, doesn't go any further (there's a ski jump that stops it).

    Later, when the car wants it to be n steps open, it pulses n steps in the opening direction, having assumed the valve was closed to start with.
     
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  15. donzoh1

    donzoh1 Active Member

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    I'm wondering whether the car I've called PriTanic might be haunted. Today, I was watching the Power Management Display in low speed driving. There were times when the ICE made revving or unstable RPM sounds but was obviously running while the display indicated no ICE activity (no moving red arrows). Whaaaaat?
     
  16. ChapmanF

    ChapmanF Senior Member

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    I'm not sure how often that display polls the CAN bus for the information it displays. I've always regarded it as more toward the "pretty graphics to entertain the passengers" end than the real-time-accurate end of the spectrum.
     
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  17. donzoh1

    donzoh1 Active Member

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    Makes sense. I drive Uber and sometimes show that screen to passengers. Many times, my gen 3 ICE is so quiet at low rpms, it can't be noticed so I wouldn't know if the display was correct or not regarding ICE operation. Now, it has the rough idle which I'm thinking means vacuum leak (tho no P0456) or could be part of the egr test mode?
     
  18. ChapmanF

    ChapmanF Senior Member

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    P0456 would pertain to a leak in the evap emissions system.

    A more general intake vacuum leak wouldn't give you an evap-specific code like that; you might be seeing P030x misfire codes, P0171 lean mixture, possibly something about MAF sensor range/performance if the leak was big enough.
     
  19. donzoh1

    donzoh1 Active Member

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    Today I did put some smoke into the air intake circuit. The O-ring on the MAF sensor was enlarged/stretched creating a leak there. It's much smoother on startup but I've not driven it. I'm assuming a substantial leak could screw up the EGR test mode? I didn't look at fuel trims or much else and no other codes are present...just P0401.
     
  20. donzoh1

    donzoh1 Active Member

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    I drove 200 miles today (in a single drive) and it's up from 39 to 47 mpg. Check engine light is off but the scanner still shows the P0401 code. I'm hoping it's going thru it's test series. The ICE was very smooth with no hesitation or bobbling.
     
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