which 02 sensor to buy

 

Which one do you need? Left, right, upper, lower, front, rear? Click on the link and scroll down to #16 and #17. That link assumes you are one of the smart folks with a 3.5L EcoBoost engine. If you have a different engine, the sensors will probably be different.

https://parts.autonationfordwhitebea...ber=be5z9g444a

Here's that same emissions parts chart for the 5.0L engine:
https://parts.autonationfordwhitebea...ber=bl3z9g444a