Ford Mustang 1999-2005

Heated Oxygen Sensor



The oxygen sensors supply the 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).

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Fig. Cross-sectional view of a common heated oxygen sensor mounted in the exhaust system

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. When testing the oxygen sensor voltage signal, it should correspond to the values shown in this chart

  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 (20°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 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.