Nissan Pick-ups and Pathfinder 1989-1995

Oxygen Sensor



The three-way catalytic converter, which is capable of reducing HC, CO and NOx into CO 2 , H 2 O, O 2 and N 2 can only function as long as the air/fuel mixture is kept within a critically precise range. The Mixture Ratio Feedback System (MRFS) is what keeps the oxygen range in control. By receiving feedback from the oxygen sensor, the control unit can determine how well the air/fuel mixture is burning and signal the engine to adjust for different driving situations.

The sensor, located in the exhaust manifold, or other part of the exhaust system, senses the oxygen content present in the exhaust gases. Basically, the oxygen sensor system works like this: When the sensor reaches a predetermined temperature, (usually around same time that the engine reaches normal operating temperature), the signal should fall within certain parameters. At this point, the computer begins to accept information from the oxygen sensor. Up to this point, the computer had ignored oxygen sensor signals, favoring instead the information from other sensors in the engine compartment. The O 2 sensor produces a small voltage level, between 0 and 1 volt, that varies depending on the amount of oxygen in the exhaust. This voltage read by the computer as oxygen content in the exhaust stream, which is an indication of combustion. The computer works together with the fuel injectors to vary the air/fuel mixture based upon this information.

If the amount of oxygen in the exhaust system is low, the mixture is said to be rich, and the sensor voltage will be high, somewhere around 1 volt. The higher the voltage signal sent to the computer, the more the computer will attempt to reduce the amount of fuel available to the engine. The amount of fuel is controlled until the amount of oxygen in the exhaust system increases, indicating a more lean mixture. When the mixture is lean, around 0 volts, the sensor will send a low voltage signal to the computer. The computer will then increase the availability of fuel until the sensor voltage increases again and then the cycle will start all over. The computer will continue this cycle in an attempt to maintain the stoichiometric air/fuel ratio of 14.7:1 which results in the most efficient catalytic converter operation.


The oxygen sensor is installed in the exhaust system between the exhaust valves and the catalytic converter. It is often placed close to the engine for quick and efficient warm-up.

The oxygen sensor is removed in the same manner as a spark plug. Exercise care when handling the sensor. Do not drop or handle the sensor roughly. When installing a new oxygen sensor, use care not to get the anti-seize compound (which is normally found on the sensor's threads) on your fingers. More importantly, make sure you do NOT get any on the probe end of the sensor. If reinstalling an old oxygen sensor, you must reapply the anti-seize compound to the sensor.

  1. Disconnect the negative battery cable.
  3. Unplug the wiring connector leading from the O 2 sensor.
  5. Unscrew the sensor from the exhaust manifold.

To install:
  1. If not already on the sensor, coat the threads of the replacement sensor with a nickel base anti-seize compound. Do not use other types of compounds, since they may electrically insulate the sensor.
  3. Install the sensor into the exhaust system. Torque the sensor to 18-25 ft. lbs. (24-34 Nm). Connect the electrical lead. Be careful handling the electrical lead as it is easily damaged.
  5. Reconnect the battery cable.


  1. Locate the sensor in or near the exhaust manifold and access the wire harness.
  3. With the engine running at normal operating temperature, unplug the connector and see if the engine idles at a different rate. If the sensor is functioning correctly, the idle should change.
  5. With the engine still running, and the oxygen sensor connected, use a multimeter and check the voltage level between the control unit and the sensor. The correct voltage should be between 0 and 1.0 volts.