Red triangle, loud fan, HV battery not charging

I posted in the main Gen II forum trying to find a Prius mechanic local to me so I can get the proper codes to diagnose my issue (New Orleans area mechanic or Techstream owner? | PriusChat), but I figured I'd lay out the problem here and see if anyone has any advice for what I can try on my own in the mean time. Any guidence would be MUCH appreciated!

Background:

So I left my 2005 w/ 228,000 mi sitting for 2 months without much shade in the Louisiana heat and I'm afraid I may have damaged the HV battery. Stupid, I know, but this is my first Prius and only my second car, so I'm just now finding out about trickle chargers and the potential of the car getting too hot while parked.


The Problem:

- Red Triangle, Check Engine light, and an orange exclamation point in a circle with parenthesis (ABS?).

- HV fan is always on HIGH when in Ready Mode.

- The display shows the HV battery not charging. (Stays on one purple bar.)

The scanner at AutoZone pulled these codes...
- C1241: Low Power Supply Voltage Malfunction https://www.autocodes.com/c1241_toyota.html
- C1259: HV System Regenerative Malfunction https://www.autocodes.com/c1259_toyota.html
- C1310: Malfunction In HV System https://www.autocodes.com/c1310_toyota.html

From what I understand, this is likely incomplete information.


What I've tried so far...

1. The 12 volt was dead from sitting, and wouldn't hold a charge, so I replaced it.

(BTW, since I've had the car, the 12v has always read a few volts lower than it's supposed to with IG off or IG on, but just fine when it's in Ready Mode. It never threw any codes, so I didn't think much of it. However I just noticed something now when I reconnect the battery. I'll connect the positive terminal first, then when the negative side makes contact with the ground, it sparks. Does that mean something is drawing current that isn't supposed to while the car is off? Or is that normal?)

2. Disconnecting the 12 volt clears the lights and the above issues go away for about 20 min, then it all comes back.

3. Decided to check the HV fan. I had driven through the desert last summer and it was CAKED in dirt/dust, so I cleaned it.

4. Checked the HV battery case and chunks of the caked dirt had made it's way onto the battery too . Vacuumed it the best I could and thoroughly wiped out all the plastic ducts.

5. Visually inspected all the connections and fuses I could think of. Didn't notice anything weird.

(People have reported leaks in the back hatch on this model leading to wire corrosion, but it's rained pretty hard a few times this last week, and everything has stayed dry as far as I can tell.)

My worry is that 3 and 4 weakened the HV battery and it sitting for 2 months in the heat killed it, but I'm holding out hope it's something simpler/less expensive. Maybe if I'm lucky, I can find someone to help replace a just a few of the cells and that'll fix it.


Thanks for taking the time to read this.