I just got my upper rear (bank 1) O2 sensor from Autozone for $65. It's a Bosch unit. BTW, the Ford unit was $140 at the dealer. And I won't even get into what they wanted for labor :rolleyes: .
You also need 2 of each. For both the front and backside of the motor.
No, not unless more than 1 sensor crapped out on you. Usually only 1 sensor will fail at a time. The OBDII codes will tell you which one.