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Check Engine Light: How to Diagnose

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Rebound, Jul 28, 2020.

  1. Rebound

    Rebound Senior Member

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    Very few amateur mechanics understand how to diagnose a Check Engine Light.

    Most people will tell you to read the diagnostic codes and proceed accordingly: A MAF Sensor error means a bad MAF sensor, an Oxygen Sensor error means bad Oxygen sensor, etc. THEY’RE WRONG. Lots of people replace working Oxygen sensors, reset the code, and the problem comes right back, because it was never solved at all.


    Others will say, “Oh, you have a P0402 error? I had one, and replacing the right-threaded helical gobsmacker fixed it.” They are also wrong. It may be that you have the same problem, or maybe not.

    The Check Engine Light means that you need to read the engine diagnostic codes. THEN, you need to perform a diagnosis, and to do that, you need to understand how the system works and what the diagnostic data is telling you. The codes are DATA, not diagnosis.

    It’s all about fuel trims. If you want to understand what’s really going on, there are two YouTube videos which will get you started:




     
    #1 Rebound, Jul 28, 2020
    pjksr02, Ed Beaty and gliderman like this.
  2. pjksr02

    pjksr02 Active Member

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    Very true. There's a whole picture to look at.

    The collective knowledge and experience of board-members, like here in PriusChat, is invaluable, too!

    (Case in point, the cylinder misfire codes.)
     
    #2 pjksr02, Jul 28, 2020
  3. Rebound

    Rebound Senior Member

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    They are GREAT videos. The guy is very clear and he teaches very well. I learned a lot from it.
     
    #3 Rebound, Jul 28, 2020
  4. ChapmanF

    ChapmanF Senior Member

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    Thanks for this post. There are lots of things that can never be repeated too many times:

    That's one of them.

    That's another one.

    And that's another.

    That one starts to need more specificity. It's about fuel trims if the diagnostic codes you got have something to do with mixture or fuel trims. There are a lot of codes the ECM can give you that aren't about fuel trims. If you get a code about, say, the camshaft position ... that won't be about fuel trims.

    It all goes back to the third point ... just doing the homework of looking up what the code means and how to diagnose what caused it.
     
    #4 ChapmanF, Jul 29, 2020
  5. Rebound

    Rebound Senior Member

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    Yes, you’re right... I guess what I meant is that a lot of the diagnosis involves understanding fuel trims and how most of the check engine related code can be diagnosed by understanding how the system combines input to adjust fuel trim. But you’re right, a CEL problem isn’t always related to the fuel trim values.
     
    #5 Rebound, Jul 30, 2020
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