P3125 and no start, 2003 Prius

Long story short, I purchased a 2003 Prius for $500 on Wednesday (8/7/24) in a brick condition. The car had no power whatsoever, did not start or do anything. The previous owners claimed that the 12v died and they replaced it and now the car was a brick. It seems as though the hooked the battery up backwards or jumped it incorrectly or something, causing a number of issues. The only thing that worked was the map light. Made the very excellent decision to purchase the vehicle and towed it home.

Started as anyone does with weird electrical issues, at the fusebox(es) and 12v battery. The 12v was dead and determined to have a dead cell after attempting to charge. There were 4 fuses blown, consisting of the main 100a fuse, the 30a RDI fuse, the 15a AM2 fuse, and the 15a THRO fuse.

I replaced all of the blown fuses (the 100a one was so much fun), tossed in an ill-fitting placeholder battery from another vehicle and the car fired right up and seemed to run well. At this point I noticed that the MFD turns on but doesn't display anything relating to flow of electricity and is locked at 99.9 mpg. The outdoor temp section is also just blank. The radio does not function and neither does the clock or anything other than climate control in the center control area. Based on my research this seems like a bad gateway module, of which I have another to put in. This, however, is just the beginning. I was able to drive the car around my driveway with no issues.

After the car warmed up, the red triangle and check engine lights came on. I used my cheapo auto parts store scanner to scan the computer and it did not show any codes. I attempted to restart the car to see if the issue persisted, which it did. I scanned the computer again which once again read nothing. After a quick google, it seemed as though this could be caused by low oil or coolant. The oil level was good, but the coolant was quite low so I assumed that was the issue. After topping the coolant off the next day I attempted to fire the vehicle up again, which it did.

I did have the 12v disconnected overnight, which seemed to clear the codes and things were well for a bit until the check engine and red triangle lights came on again, only this time, the motor revved up and the car seemed quite unhappy in general. I scanned the computer before shutting it down, I scanned the computer again to which I saw P3125, inverter malfunction. This was the last time the car ran. I attempted to restart it to see if I could move it to a better place, but to no avail. The car makes a click or two and says "READY" for a moment before failing to start and giving the red triangle and check engine with P3125.

So, I did quite a bit of research and planned on doing more proper diagnosis, until yesterday morning I was in a gambling mood and found another 2003 at my local pick and pull with only 85k (mine has 160k). I took it's inverter for $100 and installed it the same day to which I am greeted with identical symptoms (oh well, that was the gamble). So far, I have rechecked all fuses and tried a different 12v which got me nowhere.

Today, I ordered a Veepeak OBDCheck BLE + scanner and purchased Torque Pro, downloaded Dr. Prius, and Hybrid Assistant. I could not figure out how to make Torque Pro read the sub codes on the first gen that I have so any help there would be great. Dr. Prius and Hybrid Assistant gave me errors trying to read the data coming from the car, and both gave me N/A values for all of the hybrid things. All other readings worked correctly. Not sure if this is user error or an issue with possibly the HV ECU?

I was hoping to avoid Techstream since I was having issues getting my vm to work, though I can work that issue if necessary.

Any assistance would be great. Really hoping this isn't one of the MG's. I'd like to diagnose properly before partaking in more gambling. I have yet to check the individual cells of the traction battery with my multimeter.

 

Forgot to mention that I had driven the car less than half a mile total and had it running for less than 45 minutes total over the multiple times it ran.