Samiii

Hi fellows I am owning Toyota Aurion sportive 2006, I am facing issue it's engine light comes on I put the scanner and got the code P0420 catalyst system efficiency below threshold bank 1 as my research it's O2 sensor if I am not wrong but can't not locate which sensor is bank1 as it have 4 sensors is any one in this forum can help me out so I will get straight to change that sensor. Thanks in advance