Engine Exhaust OxygenSensors

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This 2001 Ford ExplorerSportTrac & V6~4.0L~SOHC engine has 3 OxygenSensors; they are 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.
A Bosch 15719 with 24i00 0m610 Long Cable+Plug is good to replace any Ford 4WireNarrowBand Sensor.
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.

OxSensors degrade+fail with time or use; replace every 10Yrs\100kMiles to maintain engine performance:
these also often fail when their Cabling\Harness has been installed too near Moving or HotExhaust parts.
Driving with a failed OxSensor can eventually cause a CatConvertor to get clogged & an expensive 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.

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