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P0401 - Check Engine Light (entire system cleaned and new EGR Valve)

Discussion in 'Gen 3 Prius Technical Discussion' started by Rob Cameron, Jul 5, 2024 at 2:38 PM.

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  1. Rob Cameron

    Rob Cameron New Member

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    Location:
    Charlotte, NC
    Vehicle:
    2012 Prius
    Model:
    III
    So, I have what appears to be a bit different issue with the P0401 Check Engine Light coming on. To hopefully make this easier, I am going to list items in bullet point:
    • 2012 Prius getting "rattle" at idle.
    • pulled and totally went through EGR system.
    • Full clean of all tubes, EGR cooler, Air Intake (pool of oil in it), etc.
    • All system components cleaned, and air/water flow checked.
    • NEW EGR VALVE installed.
    • New PCV valve installed.
    • I also installed and Oil Catch can for the oil blow by from the PCV valve, so it doesn't get to the air intake.
    • Re-installed everything and car ran great, no codes, etc.
    A year (or so) and about 5K-8K miles has been put on the car and the P0401 code and check engine light is coming back on. Car is NOT "rattling" at idle. I can Clear the code, but after driving it a few times (not assure if the number of "times" driven or number of miles, but after a while the P0401 and associated engine light on the dash will come on. Again, I can clear it and it stays away for a period of time, but eventually comes back.

    Any thoughts? I just cleaned the oil catch can out as well. I am at a loss of what the issue many be. A sensor I missed cleaning or replacing?

    Thanks in advance for any assistance you can provide!
     
    #1 Rob Cameron, Jul 5, 2024 at 2:38 PM
  2. StarCaller

    StarCaller Senior Member

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    Location:
    Cedar Crest, NM, USA
    Vehicle:
    2014 Prius
    Model:
    Two
    "What does the P0401 OBD-II diagnostic code mean? Also known as insufficient EGR flow, this particular OBD-II trouble code indicates the engine control module (ECM) has detected the exhaust gas recirculation (EGR) valve is not allowing sufficient flow of recirculated exhaust gases into the engine."
     
    #2 StarCaller, Jul 6, 2024 at 4:43 PM
  3. ChapmanF

    ChapmanF Senior Member

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    Mar 30, 2008
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    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    What any Toyota trouble code means is what is written in the DTC Detection Condition box in the section for that code in the repair manual. So for this one, it's:

    [​IMG]

    In other words, there is a self-test the ECM runs sometimes, by sneaking it in when the engine is turning with the fuel cut off (as when going down a hill). Under those conditions, the ECM can open the EGR valve a little, which should make the Manifold Absolute Pressure reading go up, and then close the valve, and the MAP reading should go down again.

    The difference (MAP reading with opened valve − MAP reading with closed valve) is saved as the result of the test. The most recent saved result can be checked, as seen in this thread.

    If that test result gets too small (opening the valve doesn't make enough difference), the P0401 code is set. So it might mean the valve isn't opening enough, or it might mean there's clogging somewhere, or it might mean the MAP sensor is wonky. Knowing how the test is done often helps in troubleshooting what might have caused the failing result.

    Other sources on the internet that offer to say what a trouble code 'means' are often not as helpful as looking up the code's actual detection condition.
     

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    #3 ChapmanF, Jul 6, 2024 at 6:41 PM
    Brian1954 likes this.
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