Heated Oxygen Sensor (HO2S)


The oxygen sensors supply the PCM (computer) with a signal that indicates a rich or lean condition during engine operation. This input information assists the computer in determining the proper air/fuel ratio. A low voltage signal from one or more sensors indicates too much oxygen in the exhaust (lean condition) and, conversely, a high voltage signal indicates too little oxygen in the exhaust (rich condition). 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. A faulty oxygen sensor due to loose connections, bad grounds, high resistance in the circuit, or opens in the circuit can cause the following symptoms.


The PCM sends a bias voltage of approximately 450 mV to the oxygen sensor. At operating temperature the oxygen sensor signal varies between 0 and 1,000 mV. When the mixture is rich the oxygen content is low and the voltage signal will remain on the high side of the 450 mV mid-range. When the mixture is lean the oxygen content is high and the voltage signal will remain on the low side below the 450 mV mid-range. The oxygen sensors on later model vehicles are equipped with a heater circuit. The heater circuit in the oxygen sensor shortens the time required for the sensor to reach operating temperature and provides a more accurate signal.

Oxygen Sensor Related Symptoms

  • Surging at idle
  • Unstable idle
  • Running rough off idle
  • Hesitation
  • Stumble
  • Chuggle
  • Poor fuel economy
  • Spark knock
  • Stalling on acceleration

Oxygen Sensor Testing

Do not pierce the wires when testing heated oxygen sensors, as this can lead to wiring harness damage. Backprobe the connector to properly read the voltage of HO2S.When testing the oxygen sensor voltage signal, it should correspond to the values shown in this chart

Oxygen Sensor Voltage Chart

  • Disconnect the HO2S.
  • Measure the resistance between PWR and GND terminals of the sensor. If the reading is approximately 6 ohms at 68°F (20°C), the sensor's heater element is in good condition.
  • With the HO2S connected and engine running, measure the voltage with a Digital Volt-Ohmmeter (DVOM) between terminals HO2S and SIG RTN (GND) of the oxygen sensor connector. If the voltage readings are approximately equal to those in the table, the sensor is okay.

Oxygen Sensor Removal

  • Disconnect the negative battery cable.
  • Raise and safely support the vehicle on jack stands.
  • Disconnect the HO2S from the engine control sensor wiring.

    NOTE: If excessive force is needed to remove the sensors, lubricate them with penetrating oil prior to removal.

  • Remove the sensors with a sensor removal tool, such as Ford Tool T94P-9472-A. To remove an HO2S, detach the vehicle wiring harness connector from the sensor wiring harness . . . then remove the sensor from the exhaust system

    Oxygen Sensor Installation

  • Install the sensor in the mounting boss, then tighten it to 27–33 ft. lbs. (37–45 Nm).
  • Reattach the sensor electrical wiring connector to the engine wiring harness.
  • Lower the vehicle.
  • Connect the negative battery cable.
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