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

Sensor11+Sensor21, pic below, are between Rght+LeftManifolds & Rght+LeftCatConvertors, respectively.
Sensor12, pic below, is mounted in between the two collective\common\downstream RearCatConvertors.
All these Sensors are operationally alike & materially similar, excepting the longer or shorter 4WireCables.
Each has a FordStandardPlug+4WireCable with Blk+Gra wires for the signal & Wht+Wht wires for a heater.
Each Sensors Heater element varies 3~9ohms & is used to decrease warmup time for the Signal element.
The upstream Sensor signals to the PowerControlModule\PCM are for ClosedLoop Air~Fuel Ratio control.
The downstream Sensor signal to the PCM monitors the CatConvertors & ensures Emissions compliance.
The PCM monitors+analyzes signal amplitude+frequency for FuelInjector control & setting TroubleCodes.
The 0.10~0.90vDC signals alternate every few seconds, responding to varying Lean~Rich Air~Fuel ratios.
An OBD2 scanner can be used to view\graph related codes & signals; I use OBDLink# MX+ & EX modules.

These Sensors degrade+fail with use; replace every 100kMiles to maintain engine+exhaust performance;
they also often fail when their cabling has been routed too near an exhaust Manifold\Pipe\CatConvertor.
Driving with a failed OxygenSensor will eventually cause clogged up CatConvertors & a very costly repair.
Sensor replacement is relatively simple. Presoaking Sensor threads at the pipe bungs with penetrating oil for a few hours can ease removal; a MapGas torch can heat & break free excessively crusty bung threads. Nimble fingers also helps to disconnect+reconnect the wiring: Sensor11's plug is on the Engines rightrear, Sensor21's plug is above the Transmissions left ShiftCable, Sensor12's plug is on the Transmissions right.

Links To Related Info

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

https://web.archive.org/web/20170701123657if_/http://www.partsgeek.com/mmparts/the_ultimate_guide_to_the_oxygen_sensor_for_dummies.html
https://web.archive.org/web/20200907142452if_/http://secure.lambdapower.co.uk/TechNotes/Tech-1.asp
https://web.archive.org/web/20200907135224if_/http://www.bosch.com/stories/40-years-of-bosch-lambda-sensor
https://web.archive.org/web/20200907151720if_/https://www.freeasestudyguides.com/heated-oxygen-sensor.html
https://web.archive.org/web/20200907151127if_/http://www.fuelinjectedford.com/page31.html

https://web.archive.org/web/20130626102106if_/http://www.motorcraftservice.com/vdirs/diagnostics/pdf/obdsm109.pdf#page=24
https://web.archive.org/web/20200822182035if_/http://troubleshootmyvehicle.com/ford/4.0L/testing-the-oxygen-sensors-1
https://web.archive.org/web/20200822182408if_/http://troubleshootmyvehicle.com/ford/4.0L/testing-the-oxygen-sensors-2
https://web.archive.org/web/20200822182457if_/http://troubleshootmyvehicle.com/ford/4.0L/testing-the-oxygen-sensors-3
https://web.archive.org/web/20200910195723if_/https://www.ranger-forums.com/4-0l-ohv-sohc-v6-tech-33/how-many-o2-sensors-137557 OxygenSensor Locations
https://www.ranger-forums.com/general-ford-ranger-discussion-15/downstream-o2-sensor-voltage-164365 OxygenSensor Diagnosis
https://youtu.be/afio03Sv_MU?t=0 OxygenSensor12@RearTailPipe Diagnos

https://youtu.be/QiA8uksEYHo?t=25 OxygenSensors Replace
https://youtu.be/9QzD8_srbSM?t=1 OxygenSensors Replace
https://youtu.be/7Rtw8jI9quA?t=40 OxygenSensor11@RghtManifold Replace
https://youtu.be/3reiaSERIXY?t=162 OxygenSensor11@RghtManifold Replace
https://youtu.be/BVUzDz6i_bw?t=24 OxygenSensor12@RearTailPipe Replace
https://web.archive.org/web/20200907182450if_/https://www.explorerforum.com/forums/threads/how-to-install-new-upstream-o2-sensors.282979 OxygenSensor11+21@Rght+LeftManifolds Replace

https://www.boschautoparts.com/en/auto/oxygen-sensors/premium-oxygen-sensors?partId=13117 OxygenSensor11+21@Rght+LeftManifold Bosch# 13117
https://www.boschautoparts.com/en/auto/oxygen-sensors/premium-oxygen-sensors?partId=15716 OxygenSensor12@RearTailPipe Bosch# 15716

https://web.archive.org/web/20201113152331if_/https://images.carid.com/walker/items/schemes/13-03880.jpg ExhaustSystem PartsList Walker
https://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
https://www.google.ca/search?q=DEC+FOR9T20432&tbm=isch&ved=2ahUKEwiCxont9v_sAhX5Ap0JHdkPAhEQ2-cCegQIABAA&oq=DEC+FOR9T20432&gs_lcp=CgNpbWcQA1D-4BdYp5EYYNSVGGgBcAB4A4AB3AKIAeUYkgEIMC4xOC4yLjGYAQCgAQGqAQtnd3Mtd2l6LWltZ7ABAMABAQ&sclient=img&ei=qLKuX4LSOfmF9PwP2Z-IiAE&bih=612&biw=1366 CatConvertors UpStream.50StateCompliant DEC# for9t20432
https://www.google.ca/search?q=walker+exhaust+53500&tbm=isch&ved=2ahUKEwjzpceq6__sAhV0A50JHbwfBLAQ2-cCegQIABAA&oq=walker+exhaust+53500&gs_lcp=CgNpbWcQAzoECAAQGFCDwCRYiNMkYOrfJGgAcAB4AIABhgGIAckGkgEDMy41mAEAoAEBqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=lKauX_OLKPSG9PwPvL-QgAs&bih=612&biw=1366 CatConvertors DnStream.47StateCompliant Walker# 53500
https://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
https://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
https://www.therangerstation.com/forums/index.php?threads/diagnosis-of-a-clogged-catalytic-converter.188268 CatConvertors Clog
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