02 sensor trouble code p1151

[Charming007]

I have a 98 mustang Gt and recently got this trouble code:

P1151 - Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 2

Does this mean i have to replace the sensor?? If so which one left or right?

Is it better to replace both upstream sensor at the same time? Or all four? \

Also what does " Bank No.2 " mean?

Thanks any info would be great