I am trying to change the passenger upstream O2 sensor in my 2013 F-150 FX4 Ecoboost. I found the location of the sensor but I cannot find where it leads to and where it connects to. Any help would be appreciated.

 

Probably buried behind the engine near the firewall. Follow the wire with your hand until you feel the connector.
.

 

The wire heads rearwards, and goes above a heavy wire harness, and is almost hidden from view.

You will see a nice wrist-cutting heat shield above the catalytic converter on that side, and you'll need to take a big Channellok and bend the edge down so you can reach past it,
then reach up and you'll find the connector for the O2 sensor on the top of it. cut the sensor wir, then rotate the wire harness enough so that you can see the connector clipped into
the harness.

Then pull it off, and you can unclip it.

It's kind of a pain.

I just did mine - $66 at Autozone for the BOSCH sensor, then a $10 off coupon for signing up for emails brings it down to $56, and
you also get a $20 gift card by mail, making the part $36.

 

it probably fell in behind the engine

 

Did you replace by going under truck or from inside fender well?

 

depends on where it fell and how far back. you can do it either way if possible