TRIANGLE OF DEATH ON PRIUS 2004 BASE

Hi everyone. Got 2004 Prius base. Drove for 4 years no problem. It has 120k now. About 6 months ago got oxygen sensor light on. Ignored it. Everything was fine until recently after moving from California to Iowa started getting triangle of death a few seconds after starting it up in the morning. When started up engine would run at first but after about half minute would stop and triangle would pop up along with "problem" sign on monitor.
Maybe worth mentioning that i started to use ethanol fuel right before the sigh began popping up.
So after 2 or 3 times shutting it down and starting it up again engine would start and run all day and at some point the sign will go away on its own. Then after a week or so it got worse. It will run for a few seconds then shut off and run on battery only draining it very fast. Then would come to a halt with no READY on display not letting to put it in gear and only letting to shut down with foot off brakes. Took it to mechanic. He replaced 12v battery. It ran for 2 days then same death sign story happened again. Also when turning and braking the sign would pop up for a second then go away.
Some other minor issues i had all along: incorrect fuel gauge readings if tank is not full. Warm air from vents with heat off.
Any help appreciated. Thanks. Alex

 

Thanks for respond. You right about engine light. It turned out to be for oxygen sensor. Mechanic said the only code he gets on his tester is oxygen sensor one and one for low hybrid battery charge. His opinion is that hybrid battery is bad and needs replacement. Those are all the codes that his tester shows.

 

I mean big battery that runs the motor. He already replaced 12v one, helped for only 2 days. I ll get the codes tomorrow. Thanks.

 

And no i didnt replace spark plugs or performed any maintenance except oil change.