O2 Sensor problems of the 2013 Toyota Prius

I keep getting a p0136 bank 1 sensor 2 error code. Took it to several mechanics and they switched the 02 censor (downstream). Also shows that bank 1 sensor 2 has 0% voltage. Unplugged the downstream 02 sensor and got a reading but no reading when plugged in. Checked wiring harness to ecm and everything was fine. Both mechanics have determined that the issue is with the ecm. My vehicle has roughly about 70k miles on it. My car will not pass smog until this is fixed. Mechanic is recommending reflashing ecm or replacing it. P0136 comes up as pending and will stay permanent on the second drive cycle. Check engine light stays on.

Engine warning light at start up came on with no degradation of operation. Car was taken to originating manufacture dealer for diagnosis. When car was purchased as certified pre-owned the sales rep upsold extended 3rd party warranty coverage that they said 'would cover everything including diagnosis labor fees. ' when vehicle was diagnosed from dealer service dept they said it was faulty O2 sensor, caused 'by rodent damage to cable'. No validation or evidence of this was provided; cable/parts not presented for inspection to customer. Customer was told lack of sensor would result in reduced gas milage. Customer asked about safety of vehicle and was told vehicle was safe to drive without part repair. Customer asked if repair was covered under warranty and was told that it was not. Customer declined repair at that time; customer was forced to pay ~$150 labor charge for inspection. Customer returned at a late time to complete repair. Again no evidence of part failure was presented and repair cost ~$400 was forced to be paid. Customer then found extended warranty coverage documents and called them to ask why repair was not covered as program documentation and contract seemed to include coverage of this issue; provider noted that dealer did not call to authorize repair at no cost to customer and customer should submit paperwork to provider for review. Customer further researched on internet the issue and found multiple reports of faulty wiring and is concerned for safety of operation due to manufacture material flaws that allow animal damage to critical wiring of car.