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P0172 and P0420 - Unmetered Air?

Discussion in 'Gen 2 Prius Main Forum' started by oldstock79, Aug 25, 2024 at 11:53 AM.

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  1. oldstock79

    oldstock79 New Member

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    Vehicle:
    2007 Prius
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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 at 11:53 AM
  2. mr_guy_mann

    mr_guy_mann Senior Member

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    Vehicle:
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    Four
    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 at 3:15 PM
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