1996 Ford Mustang DTC P1131 O2 sensor

I've been having trouble locating this sensor. The reports says as follows... Performed Electrical engine control test found code 1131 O2 sensor running lean and not switching. I just need to know where this sensor is located on my mustang since they told me it has 4 sensors total. I want to buy the sensor and intalled myself. Please Help.

Thank you,

Ricardo Barraza