Lincoln Aviator, Ford Explorer, Exp Sport, Exp Sport-Trac, Mercury Mountaineer 2000-2005

Heated Oxygen Sensor



The oxygen sensor supplies the computer with a signal which indicates a rich or lean condition during engine operation. The input information assists the computer in determining the proper air/fuel ratio. A low voltage signal from the sensor indicates too much oxygen in the exhaust (lean condition) and, conversely, a high voltage signal indicates too little oxygen in the exhaust (rich condition).

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Fig. Typical Heated Oxygen Sensor (H2OS)-most vehicles are equipped with multiple sensors

The oxygen sensors are threaded into the exhaust manifold and/or exhaust pipes on all vehicles. Heated oxygen sensors are used on all models to allow the engine to reach the closed loop faster.

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Fig. Typical Heated Oxygen (HO2S) sensor mounting locations-Ranger shown, other models are similar


  1. Disconnect the HO2S.
  3. Measure the resistance between PWR and GND terminals of the sensor. If the reading is approximately 6 ohms at 68°F (1°C). the sensor's heater element is in good condition.
  5. With the HO2S connected and engine running, measure the voltage with a Digital Volt-Ohmmeter (DVOM) between terminals HO2Sand SIG RTN(GND) of the oxygen sensor connector. If the voltage readings are swinging rapidly between 0.01-1.1 volts, the sensor is probably okay.