This 2001 Ford Explorer Sport Trac with V6~4.0L~SOHC engine has 3 OxygenSensors; they are also known as HEGO\HeatedExhaustGasOxygen, H02\HeatedOxygen, Lambda\NarrowBand Sensors; located as shown below.

Sensor11+Sensor21 shown below, are between the Rght+LeftManifolds & Rght+LeftCatConvertors respectively.
Sensor12 also shown below, is mounted in between the two collective\common\downstream RearCatConvertors.
All 3 Sensors are operationally much alike & materially simular, excepting for the longer or shorter 4WireCables.
Each Sensors FordStandardPlug+4WireCable has Blk+Gra wires for the signal & Wht+Wht wires for the heater.
Upstream Sensors have ~9ohm heaters, the downstream Sensor has a ~4ohm heater to decrease warmup time.
The upstream Sensors send signals to the PowerControlModule\PCM for ClosedLoop control of Air~Fuel ratios.
The downstream Sensor signal to the PCM is for monitoring the CatConvertors, ensuring Emissions compliance.
The PCM monitors & analyzes signal amplitudes & frequencies for FuelInjector controls & setting TroubleCodes.
The 0.10~0.90vDC signals alternate every few seconds, corresponding to changing Lean~Rich Air~Fuel ratios.
An OBD2 scanner is handy to view\graph related codes & signals; I use an OBDLink.MX+ & Samsung phon.

These Sensors will Degrade\Fail; replacement every 100kMiles helps maintain engine efficiency & performance;
they often fail when wiring is melted due to being routed too near a hot exhaust Manifold, Pipe, or CatConvertor.
Driving with a failed OxygenSensor will eventually cause CatConvertor damage & much more expensive repairs.
Sensor replacement is relatively simple. Presoaking the Sensor threads at the pipe bung with penetrating oil for a few hours typically will ease removal; if this does not work, a MapGas torch can be used to heat & break loose excessively rusty\crusty pipe bung threads. Nimble & patient fingers help with Disconnecting\Reconnecting the cable to harness plugs: Sensor11's plug is to the rightrear of the Engine next to the Firewall, Sensor21's plug is to the left of the Transmission above the ShiftCable, Sensor12's plug is to the right of the Transmissions Pan.

Links To Related Info

Thanks go to the brave souls who've preceded me in similar adventures & freely shared their experience below:

http://web.archive.org/web/20170701123657if_/http://www.partsgeek.com/mmparts/the_ultimate_guide_to_the_oxygen_sensor_for_dummies.html
http://web.archive.org/web/20200907142452if_/http://secure.lambdapower.co.uk/TechNotes/Tech-1.asp
http://web.archive.org/web/20200907135224if_/http://www.bosch.com/stories/40-years-of-bosch-lambda-sensor
http://web.archive.org/web/20200907151720if_/https://www.freeasestudyguides.com/heated-oxygen-sensor.html
http://web.archive.org/web/20200907151127if_/http://www.fuelinjectedford.com/page31.html
http://web.archive.org/web/20130626102106if_/http://www.motorcraftservice.com/vdirs/diagnostics/pdf/obdsm109.pdf#page=24
http://web.archive.org/web/20200822182035if_/http://troubleshootmyvehicle.com/ford/4.0L/testing-the-oxygen-sensors-1
http://web.archive.org/web/20200822182408if_/http://troubleshootmyvehicle.com/ford/4.0L/testing-the-oxygen-sensors-2
http://web.archive.org/web/20200822182457if_/http://troubleshootmyvehicle.com/ford/4.0L/testing-the-oxygen-sensors-3
http://web.archive.org/web/20200910195723if_/https://www.ranger-forums.com/4-0l-ohv-sohc-v6-tech-33/how-many-o2-sensors-137557
http://youtu.be/afio03Sv_MU?t=0 OxygenSensor12@RearTailPipe: Diagnos
http://youtu.be/QiA8uksEYHo?t=25 OxygenSensors: Replace
http://youtu.be/9QzD8_srbSM?t=1 OxygenSensors: Replace
http://youtu.be/7Rtw8jI9quA?t=40 OxygenSensor11@RghtManifold: Replace
http://youtu.be/3reiaSERIXY?t=162 OxygenSensor11@RghtManifold: Replace
http://youtu.be/BVUzDz6i_bw?t=24 OxygenSensor12@RearTailPipe: Replace
http://web.archive.org/web/20200907182450if_/https://www.explorerforum.com/forums/threads/how-to-install-new-upstream-o2-sensors.282979 OxygenSensor11+21@Rght+LeftManifolds: Replace
http://www.boschautoparts.com/en/auto/oxygen-sensors/premium-oxygen-sensors?partId=13117 OxygenSensor11+21@Rght+LeftManifold Bosch.13117
http://www.boschautoparts.com/en/auto/oxygen-sensors/premium-oxygen-sensors?partId=15716 OxygenSensor12@RearTailPipe Bosch.15716
http://web.archive.org/web/20201113152331if_/https://images.carid.com/walker/items/schemes/13-03880.jpg ExhaustSystem PartsList Walker
http://www.google.ca/search?q=%2B%22walker+exhaust+50202%22&tbm=isch&ved=2ahUKEwi3vKPu6P_sAhVBUM0KHTvNA4kQ2-cCegQIABAA&oq=%2B%22walker+exhaust+50202%22&gs_lcp=CgNpbWcQA1DpxwFYu_gBYK79AWgBcAB4AIABtgKIAYMpkgEIMC4yNS4zLjGYAQCgAQGqAQtnd3Mtd2l6LWltZ8ABAQ&sclient=img&ei=_aOuX_fJFsGgtQa7mo_ICA CatConvertors UpStream.47StateCompliant Walker.50202
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http://www.google.ca/search?q=Bosal+079-4172&tbm=isch&ved=2ahUKEwjQlJq67v_sAhUQX80KHU9eCsIQ2-cCegQIABAA&oq=Bosal+079-4172&gs_lcp=CgNpbWcQA1CDGVjNJWCLKmgAcAB4AIABqgGIAfIFkgEDMC42mAEAoAEBqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=2qmuX9C_L5C-tQbPvKmQDA&bih=612&biw=1366 CatConvertors DnStream.47StateCompliant Bosal.719-4172
http://www.google.ca/search?q=DEC+FOR920586&tbm=isch&ved=2ahUKEwjwvc2r-P_sAhUJBs0KHUvgBqkQ2-cCegQIABAA&oq=DEC+FOR920586&gs_lcp=CgNpbWcQAzoCCAA6BQgAELEDOgQIABBDOggIABCxAxCDAVD0nhZYiLQWYPbAFmgAcAB4AIABmAGIAf8KkgEEMC4xMZgBAKABAaoBC2d3cy13aXotaW1nwAEB&sclient=img&ei=OLSuX_DBH4mMtAbLwJvICg&bih=612&biw=1366 CatConvertors DnStream.50StateCompliant DEC.for920586
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