1. Attachments are working again! Check out this thread for more details and to report any other bugs.

05' P0a08-264/P0a09-591 head scratcher

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Michael Cyr, Apr 6, 2022.

  1. Michael Cyr

    Michael Cyr Junior Member

    Joined:
    Jul 23, 2014
    9
    3
    0
    Location:
    Belgrade, ME
    Vehicle:
    2008 Prius
    Model:
    Four
    Hi all. Hoping for some pointers here because I'm stuck. I have an 05' with red triangle, ABS, Brake lights on, as well as the battery and car warning icons on the MFD. The car runs, the HV battery appears to charge, as well as the 12v battery (13.8v with engine on).

    Techstream shows P0a08 with code 264 and P0a09 with code 591. This seems related to the 12v charging circuit. The 12v battery was replaced before I got the car and it appears that the codes have been present since then. The replacement battery is a NAPA brand sealed AGM.

    I have checked a number of thing and changed the engine compartment converter.
    1. I have battery voltage on the output side of the battery compartment fusible link, both on the skinny white wire and the thick white wire. The 5amp and 100amp fuses test good.
    2. The fusible link in the engine compartment fuse block tests good.
    3. Testing at c5 & c6 harness connectors on the back of the converter shows that I have all expected voltages EXCEPT I have an open on the NODD pin on the harness and do not see the 100ohm to ground reading that is called for.
    4. I changed the converter because I had another one. no effect. I opened up on the original converter and there's no evidence of damage on top or bottom boards.
    5. I've checked the pins and connectors on the HV control ECU behind the passenger side dashboard. Connectors and pins look good.
    Should I be looking to replace the HV control ECU at this point. The manual talks about checking the NODD hardness but I don't know where to test it back to.. presumably some pin on the HC control ECU. I think the check on the C5 for voltage confirms that both 12v connections to the aux. battery are good.

    Any other troubleshooting tips for this?

    Thanks!

    Mike in Maine
     
    #1 Michael Cyr, Apr 6, 2022
  2. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    107,796
    48,996
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    #2 bisco, Apr 7, 2022
  3. Michael Cyr

    Michael Cyr Junior Member

    Joined:
    Jul 23, 2014
    9
    3
    0
    Location:
    Belgrade, ME
    Vehicle:
    2008 Prius
    Model:
    Four
    #3 Michael Cyr, Apr 7, 2022
  4. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    23,324
    15,110
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    The P0A08 code is a general one indicating some abnormal status reported by the DC/DC converter at the front of the car. Pretty much anything that interferes with the converter, causing it to shut down or failsafe, will lead to a discharging 12 volt battery and eventually to a P0A08-264 code. So its workup section in the repair manual has a bunch of things to potentially check.

    The P0A09-591 code is much more specific. It refers to the VLO circuit between the converter and the HV ECU.

    noddvlo.png

    The HV ECU has two single-wire circuits it uses to control and monitor the DC/DC converter. NODD (that violet wire, between C5 pin 4 and H16 pin 24, works bidirectionally: the HV ECU can use it to wake up or shut down the converter, but also to read some converter status back. (The details on how that works are sketchy.)

    The VLO circuit, the blue wire between C5 pin 5 and H16 pin 31, is unidirectional. The HV ECU uses it to tell the converter to lower its output voltage a little, under some circumstances.

    P0A09-591 means that VLO wire is either open or shorted to ground (from the HV ECU's perspective). You can see this is specific even about the kind of fault. If the wire were instead shorted to battery voltage, that would give a different code (P0A10-592).

    So it doesn't seem like something too difficult to pin down with a multimeter and some exploration of that circuit (from the converter connections under the hood to the HV ECU behind the glove box).

    Solving that issue might also solve the more generic P0A08.
     
    #4 ChapmanF, Apr 7, 2022
    SFO likes this.
  5. Michael Cyr

    Michael Cyr Junior Member

    Joined:
    Jul 23, 2014
    9
    3
    0
    Location:
    Belgrade, ME
    Vehicle:
    2008 Prius
    Model:
    Four
    Thanks. That is helpful. That is the direction I was going because I get an open on the NODD at the converter, instead of 100-150ohm to ground. The pin numbers and wire colors are very helpful. Mike

     
    #5 Michael Cyr, Apr 9, 2022
  6. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    23,324
    15,110
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    What do you get for VLO, which is the circuit referred to by your trouble code?
     
    #6 ChapmanF, Apr 9, 2022
(You must log in or sign up to post here.)