The oxygen sensor provides the computer with a signal indicating a rich or lean condition during engine operation. This information helps the computer determine the correct air/fuel ratio. A low voltage signal from the sensor indicates too much oxygen in the exhaust (lean condition), while a high voltage signal indicates too little oxygen (rich condition). Oxygen Sensor are threaded into the exhaust manifold or exhaust pipes on all vehicles. Some later models use heated Oxygen Sensor to allow the engine to reach the closed loop faster. For non-heated sensors, with the oxygen sensor connected and the engine running, measure the voltage between the oxygen sensor connector and ground with a Digital Volt-Ohmmeter (DVOM). If the voltage readings swing rapidly between 0.01-1.1 volts, the sensor is likely okay. For heated sensors, disconnect the HO2S, measure the resistance between PW Rand and GND terminals of the sensor. If the reading is approximately 6ohms at 68 degrees Fahrenheit (20 degrees Celsius), the sensor's heater element is in good condition. With the HO2S connected and engine running, measure the voltage with a DVOM between terminals H02S and SIG RTN (GND) of the oxygen sensor connector. If the voltage readings swing rapidly between 0.01-1.1 volts, the sensor is likely okay. To remove and install, disconnect the negative battery cable, raise and safely support the vehicle on jackstands, disconnect the HO2S from the engine control sensor wiring. If excessive force is needed to remove the sensors, lubricate the sensor with penetrating oil prior to removal. Remove the sensors with a sensor removal tool, such as Ford Tool T94P-9472-A. 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, and connect the negative battery cable.
