Check Hybrid System - After 12V drained

Wanted to start a thread in the hopes of getting some advice as I sit over the weekend contemplating my disaster of a week.

We've got a 2010 with about 190,000 miles on it. Bought it new, original owners. Almost all highway miles with about a 40 mile commute each way.

On Saturday I drove about 180 miles round trip and on the ride home I left the interior dome light on accidentally and forgot about it until Tuesday. Came out to the car and obviously found a dead 12V battery. Grabbed jumper cables and used the engine compartment jump points to start the car. Not the first time I've done this in the course of owning the Prius over the last 10 years. Get the READY message the engine attempts to start, or something similar as it didn't sound exactly right, and after about 2-3 seconds the noise stops and I get a check Hybrid System Error. Uh oh... that's odd.

So I pulled the 12V battery, disconnected the Orange breaker in the trunk and pulled the white lead from the main fuse panel under the hood. I put the 12V on a charger in the house overnight, brought it back out in the morning. Check Hybrid system error isn't there, press Start and same exact behavior. (Prius attempts to do something in the engine compartment for about 2-3 seconds then check Hybrid System error.

Run out, grab a new 12V battery, rinse.. lather.. repeat..

I've got a mini-VCI cable coming on Monday so I can read out the codes but I'm really shocked at this point how a simple dead 12V battery could cause such a failure. Any advice as to next steps and has anyone seen anything like this? I've read every thread I could find but no conclusions or root causes ever seem to be presented. Thanks in advance.

 

Sorry totally forgot to mention that.

So after the battery was recharged, hooked the 12V battery back up. Put the orange safety plug back in, made sure it was properly seated and pushed the lever to the right to ensure it was secured properly. Then plugged the white lead back into the fuse box.

Same issue, engine attempts to do something and then shuts down and displays the Check Hybrid System error.

Went out and grabbed a new AGM battery. Repeated all steps above, same issue.

 

Yes tested both old and new 12V with a multimeter and both look fine with expecting resting voltage.

 

It did come on several times throughout my initial 12V battery troubleshooting. I do know there were times when it didn't however.

 

Didn’t reverse polarity. I’ll triple check the fuses this week. I was nervous about how many times I can try and start the car and will have to research a way to charge the HV if it comes to that. Ugh. Thanks!

 

Checked all the fuses and on first go around they all look good. I've already replaced the 12V battery and charged it directly without using the front jump point. Good to know there might be someone in my neck of the woods if the HV battery needs charging. I'm on the eastern side of New Hampshire, north of Portsmouth by about 35 miles.

I can't force charge the pack at the moment as the engine does not start and appears to shut down the fuel pump after about 5-10 seconds. (Was told that by a friend who took a quick look at it)

 

So hooked up to TechStream and basically had every single ECU system throwing High/Low voltage codes. Ended up being the ECU harness in engine compartment. Latch was broken and the cable slowly worked it’s way off the computer. Plastic clip/latch is part of the cable and can’t be replaced without replacing entire cable. Industrial grade zip tie to the rescue and everything is back to normal!