What is wrong with my 2007 Toyota Prius?

Long story short, my 2007 Toyota Prius has unfortunately stopped working. For reference, the battery was replaced around 120,000 from original owner, and it is currently sitting around 205,000. I did the battery level diagnosis, and it was sitting at 11.6V. Previous to this, I had the check engine light on for a little while, only having the error P0420, which has to do with the catalytic converter/O2 sensor. I was told that it is not that big of a deal, and it only slightly messes with gas mileage. I had ordered an 90 degree extender for the O2 sensor to be put on as soon as I got back, as that was everyone's suggestion to do for this error code.

Drove it to the gym with no problem. When I was leaving, I suddenly got the infamous *red triangle of death*, where the Prius started to show a decrease in performance, went down on speed, and eventually gave out on me in a parking lot. Additionally, the VCS, parking warning light, and check engine light came on. I talked to a Toyota mechanic on the phone, and he suggested I try to put some gas in it, as I was on my last bar and it was possible that the fuel gauge sensor had been glitchy, thus I had run out of gas. However, after putting the additional gas inside of it, instead of fixing the problem, it actually CHANGED the problem. The *red triangle of death* was no longer visible, yet the VCS, parking light, check engine light, with the added indicator lights of ABS and the slip indicator (which indicates that the VCS is working, from what I have seen while researching) were shown as well. What is weird is when I held my foot on the break to imitate starting the engine/hybrid battery, it only acted as if it were turning on the electronics. When I would start it without foot on the break (to turn on electronics), and then put foot on break and press power button again, the car would shut off. To put this simply, it seems that only the accessory mode works, instead of ignition mode being able to work.

I really don't know where to go from here. It might be the catalytic converter that needs replaced, it might be the battery. It also might be the brake system? Personally, my guess would be a new battery is needed, yet I'm not sure why these other indications would be presented, as accessory mode still works and what not. I would just like some educated guesses and possibly accurate diagnoses from anyone who may have experienced this before and/or are educated mechanics/car enthusiasts.

Much thanks and sincerity in advance.

 

Thank you SO much for that extremely helpful answer to my question. The Prius is 100% fixed, thanks to this!

 

As I ran the battery test, it came to 11.6V. I was thinking the battery is the problem as well, yet the key faub still works, and accessory mode/IGNITION ON mode still work without trouble.