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P0A0F-205, but no other engine codes, diagnosis help?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Kelp, Jul 26, 2022.

  1. Kelp

    Kelp Junior Member

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    Long time browser, first time posting.
    We have a 2007 Prius base, and I am trying to diagnose a problem.

    Previously, the car has been a little "clunky" at idle; when the system switched back and forth between the gas and electric motors, the engine stops kind of abruptly. But the car drove fine with no indicator lights on.

    A few days ago we were at a stop light. The light turned green and we went to accelerate, and the car clunked and died. The red triangle, and the circle indicator light with the exclamation marks came on the dash, but no check engine light. The info display also said "problem".

    I scanned the car with TechStream, and there was no code other than P0A0F-205. No engine DTC. I am having trouble figuring out what the problem could be. I feel like I have read all the threads and service manuals about the 205 code, but i seem to hit a dead end about what exactly to test or do about the 205 sub-code. The 205 PDFs just say to get the code and then proceed to the DTC page, but the P0A0F-205 is the only code I have. The diagrams say if there is no code, then to replace the ECM. Do I replace the ECm on behind the passenger glove box?

    I was due to change plugs and coils, so I did that anyway thinking maybe it was a misfire, but I know that probably wasn't the solution to the P0A0F-205 code.

    Seems like it was an intermittent issue because the car seems to drive fine now, but I would like I know if maybe the HV ecm is on its way out or something. It would be nice to know the issue is fixed for good.

    I am happy to answer any questions or follow any diagnosis procedures. I appreciate any and all help.
     
  2. mr_guy_mann

    mr_guy_mann Senior Member

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    Did you save the freeze frame data for the P0A0F? The different INF codes for this fault might mean something to a Toyota engineer but they didn't give any details in the diagnostic write up.

    So you are looking for an intermittent fault. Could be a power or ground problem to the ECM. The engineers assume that any fault that would cause a no start MUST set a code in ECM. I not so sure about that, and I think it less likely that the ECM is bad- they can fail, but it's extremely rare.

    Posted via the PriusChat mobile app.
     
  3. Kelp

    Kelp Junior Member

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    I didn't "save" it per se, but I noted that it was the 205 INF code.
     
  4. mr_guy_mann

    mr_guy_mann Senior Member

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    I would want to see if the HC ecu had saved any ECM data in the freezeframe. RPM, load, - something that might indicate whether the ECM was functioning when the no start occurred.

    Posted via the PriusChat mobile app.
     
  5. Kelp

    Kelp Junior Member

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    I see. It did have ambient temperature saved, rpm was 0 (we were stopped at a light), run time was 75 seconds (we had just left a parking lot), but I don’t remember any of the other numbers. Apologies for not saving the entire read out. If it happens again I will be sure to save the whole FF.
     
  6. Kelp

    Kelp Junior Member

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    Can anyone help me follow the diagnostics in this service manual page? https://share.qclt.com/%E4%B8%B0%E7%94%B0%E6%99%AE%E7%91%9E%E6%96%AF%E5%8E%9F%E5%8E%82%E8%8B%B1%E6%96%87%E6%89%8B%E5%86%8Cpdf%E6%A0%BC%E5%BC%8F/repair%20manual/04pruisr/05/21bpm/0a0f204.pdf

    From what I can understand, this is the page for the P0A0F-205 code. The inspection procedure says to read the DTC's, and then proceed to the relevent DTC chart. However, the DTC I got was P0A0F-205, so I just end right back up at this page. Am I supposed to have another DTC? And if I don't, what should I do?
     
  7. ChapmanF

    ChapmanF Senior Member

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    The P0A0F is a gripe from the HV control ECU, "I asked ECM to start the engine, and that didn't happen."

    In a lot of cases, the ECM itself will have more of the information on an actual problem.

    I'm a little surprised there are no codes from the ECM listed. I might be tempted to go create some simple code for the ECM (unplug the MAF sensor, say), and make sure your scan tool shows that code. If not, then there's a little preliminary project of finding out why.
     
  8. Kelp

    Kelp Junior Member

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    That's a good idea with the MAF. I will try that at my next opportunity.
     
  9. mr_guy_mann

    mr_guy_mann Senior Member

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    That actually reminds me of a common "no code - no start" scenario. Normal use results in carbon goo building up in the intake throttle body and on the sensing elements inside the MAF sensor.

    Cleaning both is a good idea. Use brake clean or MAF cleaner spray on the 2 resistor- looking elements in the MAF (careful- those are fragile).

    Use the same cleaner and a rag + an old toothbrush to remove the black stuff in the throttle body where the blade is.
    DSC_3737_2.JPG DSC_7606.JPG

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  10. Kelp

    Kelp Junior Member

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    I have some MAF cleaner, I will try that too.
     
  11. Dxta

    Dxta Senior Member

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    So did that help?
     
  12. lkhdiagnostic

    lkhdiagnostic New Member

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    Hello, my car has the same problem, I checked everything in engine management system, fuel pressure, intake system sensors everything. problem is intermittent and when it happens Only one code is stored P0A0F in hybrid control module. There are no other problems or symptoms.
     
  13. mr_guy_mann

    mr_guy_mann Senior Member

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    Have you cleaned the throttle body bore and MAF sensor elements as mentioned above?

    What scantool are you using? When the engine (ICE) is running "normally", what are Short Term and Long Term Fuel Trim values at hot in-park "idle" (1000RPM), part throttle (say 2000-2500); and driving at steady 45 (ICE running) and 65?

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  14. ChapmanF

    ChapmanF Senior Member

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    Just like back in post #7, i would encourage some action to find out why you are not seeing other codes. P0A0F is just the HV control ECU saying it cued the ECM to start the engine, and it didn't happen. Typically there will be one or more codes in the ECM to say why.

    An easy thing to check right now is to go do pretty much anything that we know sets a code in the ECM—like leaving the MAF sensor disconnected, that'll set a P0102—and make sure the check-engine light comes on and your code scanner shows the P0102. If it doesn't, there's a little side project to figure out why.