Hey I don't own a mustang, I'm just helping out a friend with his.
We got a malfunction in the rear O2 sensors, so I figured I'd start out with replacing
the rear O2 sensor.

The car is a 1998 Mustang V6.

Question is, is P0156 for the driver or passenger side rear O2?
And where should I go to buy a new O2 sensor for cheap?

Is there anything else I should check before putting in a new O2 sensor?

 

check to make sure the wires arent burnt or cracked, some times its not the 02 but the wiring of the sensor