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P0172 and P0420 - Unmetered Air?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by oldstock79, Aug 25, 2024.

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  1. oldstock79

    oldstock79 New Member

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    Hi-

    2007 PRIUS with about 143000 Miles. I have been consistently getting P0172 and P0420 codes.

    My issues started with random misfires Cylinder 3 and 4 and car running very rough- Car was obviously running rich and may have soured the cat??

    After reading through multiple posts here (thanks Priuschat!) and understanding maintenance intervals- I replaced the following in the last 300miles

    1. Fuel Injectors(made mistake and ordered reman ones)
    2. Spark Plugs
    3. MAF sensor
    4. Upstream and Downstream 02 sensors


    Now, the idle is better but still misfires (Shaking engine)- no misfire codes through only the P0172 code and then P0420- I am using Car Scanner Pro and a Bluetooth OBD2 Connector-

    What are some of the steps to figure out where "unmetered air" is getting into the engine?


    ******When I was changing spark plugs - I did over torque the ignition coil and ended stripping the threads off IG coil 3- the boot is just barely tight but still held on good and coil firmly seated.*******


    I am trying to figure out how to post freeze frame data here- Will upload it tonight-
     
    #1 oldstock79, Aug 25, 2024
  2. mr_guy_mann

    mr_guy_mann Senior Member

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    For the moment, ignore the P0420. The cat test has no meaning if the engine isn't running correctly.

    The ECM calculates a base fuel injector on time from sensor data -MAF, RPM, ECT, and so on. It then looks at the Air Fuel Ratio sensor (in the exhaust manifold) to see if the resulting AFR is what it expects.

    If the AFR is too lean (not enough fuel) the ECM makes a positive correction (+) to fuel trim. If AFR is too rich (too much fuel) the ECM makes a negative correction (-).

    P0172 is set when the ECM has to make a (too large) negative fuel correction for too long.

    What can cause this? There can be a faulty base sensor input (MAF is OVER reporting - gram per second higher than it actually is - this is the opposite of an unmetered air leak, ECT incorrect, etc), the fuel pressure can be too high, the injectors can leak or just flow too much gas, or maybe the AFR sensor is just lying to the ECM.

    A place to start is the MAF sensor. Here are some "known good" readings from my 2006 (taken at near sea level). See what your readings are under the same conditions.
    https://priuschat.com/index.php?posts/3458294

    Posted via the PriusChat mobile app.
     
    #2 mr_guy_mann, Aug 25, 2024
  3. oldstock79

    oldstock79 New Member

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    Thanks mr_guy_mann

    I pulled the battery fuse and reconnected it to clear historic ECU codes- Now I get a P0300

    Tested MAF- We are at about 2700ft ( off it shows 0.07)

    Looked at fuel trim and LTFT is set at -11.72 - initial guess is a faulty injector or "leaky" fuel injector? How to diagnose which one is faulty?

    Should I drive around to see if there are further codes? I did not idle the car for long -
     
    #3 oldstock79, Aug 26, 2024
  4. MAX2

    MAX2 Member

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    Altitude reduces the amount of air, and therefore oxygen in the air.
    Tuned fuel systems should reduce the amount of gasoline in the mixture to maintain stoichiometry.
    Apparently, your idle speed is not adjusted.
    You need to warm up the car to a temperature above 158F (70C), turn off the car, disconnect the 12V battery for a minute to reset the settings.
    Turn on the car again. Press the gas pedal slightly and release it.
    After this, the idle speed value and settings will be updated.
     
    #4 MAX2, Aug 26, 2024
  5. mr_guy_mann

    mr_guy_mann Senior Member

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    You need to check MAF readings at a few conditions while running. My chart has several RPM and load conditions - you do the same and see what the gram per second readings are.

    I was at 700 ft elevation (BARO about 29.3" Hg), you're at 2700ft (27.3" Hg). 27.3รท29.3= about 0.93.

    So take the MAF values from my chart and multiply the numbers by 0.93 - that's what you should get for your car at the same RPM and load.

    There is less air pressure as altitude increases, so lower MAF readings (and the engine makes less power-torque).

    Posted via the PriusChat mobile app.
     
    #5 mr_guy_mann, Aug 27, 2024
    Last edited: Aug 27, 2024
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