POAFA, Engine Won't Turnover [solved]

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by ShaneBacon, Aug 9, 2021.

  1. ShaneBacon

    ShaneBacon Junior Member

    Aug 18, 2015
    2008 Prius
    Hey everyone, I had P0AFA issues with my 2008 Prius with 225,000 miles and I thought I'd share how I went about fixing it so hopefully it'll help someone in the future.

    The Issue:
    I had the car in a more humid climate for a couple of months. The car drove normally and I had no issues. One morning I went to start it up and I got the triangle of death and the engine wouldn't turn over. I've had battery issues with this car before (P0A80, fixed two times by replacing individual cells) but I've never had it refuse to turn over. With a scan gauge I was only able to read P0AFA (low hybrid battery voltage).

    After towing the car back home I bought a bluetooth OBDII scanner and downloaded the Dr. Prius app on the App Store so that I could see the block voltages without having to take apart the entire car. Never knew of this trick before, and I highly recommend everyone gets a cheap scanner (mine was $25 on Amazon) and the Dr. Prius app (free!!!) to troubleshoot battery issues.


    The app pinpointed my problem to the first three battery blocks reading 0V likely indicating it was a voltage sensor issue. After taking apart the battery, and removing the safety plug of course, I discovered LOTS of corrosion on the bus bars and voltage sensors. I'm thinking maybe the extra humidity accelerated the corrosion?


    I originally was just going to remove the corroded parts and clean them, but I found that some short caused the voltage sensor harness to burn up at the pins into the hybrid battery control module.

    tempImagectf4at.png 0284905F-88E4-404F-97BF-ABA0A0D7E6D5_1_105_c.jpeg

    I'm not entirely sure, but I think corrosion likely caused some sort of short and a lot of current to be drawn through the sensors, eventually shorting them. Maybe corrosion caused a short between the connections at the pins of the hybrid battery control module. After a new control module, bus bars, hex nuts, and voltage sensor harness (total of ~$200 on eBay) I was able to get the car to start up and clear all codes!

    Overall, I found this repair to be a way easier fix than replacing a cell and making sure the balance of the battery pack is correct. Considering how close I was to junking the car, I hope this post helps someone decide to go with the relatively inexpensive repair instead of the junk yard or an entirely new battery! Definitely make sure to check out more comprehensive videos if you're not familiar with the hybrid battery system or with high voltage systems. In my opinion, they're very safe to work on if you familiarize yourself with basic electronics and the safety procedures!

    Hope this helps!
    promises3218 and SFO like this.
  2. ChapmanF

    ChapmanF Senior Member

    Mar 30, 2008
    Indiana, USA
    2010 Prius
    That's a known thing that happens. Pretty much always starting at pin 22.

    It is a timesaver to recognize that this trouble code means a voltage reading that is implausibly low, as in, not a voltage that even a bad NiMH module should have. So seeing this code generally means there is a sense wiring or ECU repair coming up, rather than a NiMH module replacement (which would be the usual thinking for the P0A80 code instead).

    ShaneBacon likes this.
  3. PriusCamper

    PriusCamper Senior Member

    Mar 3, 2012
    Pacific Northwest, USA
    2007 Prius
    Yes, you are correct... Most common hybrid battery problem I see on Prius in Pacific Northwest. Other than a couple months in Summer we have really bad year round humidity issues. But I'd take that everyday over road salt.