ECU or Battery (or both)???

This is because Toyota is greedy on so many levels. For starters the ECU you need only costs $100 used on Ebay and Amazon, but Toyota would rather rob you of an additional $700 even though there's zero difference between new and used for this electronic item

Second, Toyota's greed have got them to stick with more profitable to them antiquated Nickel-based battery that haven't been considered advanced technology since the late 1980's. They refused to switch to lithium at scale for decades now and these awful batteries leak a tiny bit of toxic electrolyte at the seals which cause the corrosion that fried your ECU.

This is one of the most common problems with the hybrid battery when it get old. It's a pretty easy fix though, you just replace the bus bars and voltage sensor harness (Wire, Frame, No.2) and ECU. Takes a couple hours and if you or a friend or family member wants to learn how you're looking at less than $200 parts and whatever you have to pay for labor... But take it to a Toyota Stealership and they'll rob you of $1500-$3000 to do the same thing. It's so infurating!

 

Yeah I wouldn't buy the ECU You can get used ones I've got a few here they're plentiful so I wouldn't think the hybrid battery ECU would solve your problem there may be a plug problem which you can look at yourself usually the pins get nasty on the plug that goes to the hybrid battery ECU that's probably what they saw and now they're going to get you for an ECU and I guess they'll splice in a new plug or whatever they can do It's like molex pins in the plug. Check out campers advice above and look at that yourself if possible or take it to a shop that's not afraid to look at it once the orange plug is out of the battery things are pretty safe generally speaking no one's going to get killed per se less than doing something really silly