2001 Ford F-150 catalytic convertor

On a V6 there are three of them, the upstream, or the before cat, are in the exhaust manifolds, or Y pipe before the converter, then 1 downstream just after the converter.
Striaght 6 1 before and 1 after the converter.

Are you asking because you got P0171, P0174 codes? If you did, don't replace the O2 sensors, it won't help.

Dave

Six cylinder 4.0 had a plastic manifold, and it developed a leak in the seam between the two halves, spray some gumout an the manifold with the engine running, if idle changes, you have a leak, and need to seal that to get rid of the codes.

It's not impossible, what you need is a scanner that reads live sensor data, and you can monitor the 02 sensors, the upstream 02's should be constantly switching between 0 and 1 volt, and the downstream(after the cat) should remain fairly even, with 73,000 it's not usually a bad 02, but it snt impossble. 171 and 174 can be set by a vacuum leak, use TBI spray cleaner and spray around manifold and vac lines, if idle changes, you found a leak, repair all leaks, clear codes and see if they set again. The other thing about 02 sensors, if they get oil or grease on the wires, it WILL affect thier ability to perform, these sensors actually breath through the wires!