How to Check an Oxygen Sensor for a Nissan Pathfinderby Don Bowman
The oxygen sensor measures the amount of free oxygen in the exhaust system as an indicator of fuel mixture. The optimum fuel mixture is 14.5:1 for the best combustion and emission control. It creates a voltage in the millivolt range when it comes into contact with oxygen. The computer sees this signal and adjusts the on-time of the injectors to compensate. There are at least two oxygen sensors on a Nissan Pathfinder, one in front of the catalytic converter and one behind. The reason for this is to measure the amount of unburned fuel going in to the converter and the amount of emissions coming out.
Insert the connector of the scanner into the OBD port under the driver's side of the dash. Start the vehicle and press the button on the scanner for "generic OBD II."
Press the button for "data stream" followed by "deluxe display." Move the cursor down until it shows the oxygen sensors. It will indicate "BI-SI" and "BI-S2," which indicates "bank 1" and "sensor 1 or 2." The word "bank" is irrelevant in a four- or straight six-cylinder such as a jeep. It only applies if it is a V-6 or V-8, indicating each side of the motor.
Watch the activity of the two sensors. They will range from 0.5 millivolts to around 0.85 millivolts and will fluctuate rapidly. The number 2 sensor should react much slower and with much lower readings if the converter is good. If there is very little activity in either sensor and the "check engine" light was not on, the sensor indicates that the mixture is wrong. This could be caused by the state of the tuneup or some other irregularity in the engine. When the sensor is working properly, the scanner will indicate that the O2 system is in a closed loop. A light on the scanner will fluctuate from bright to dim continuously.
- The front sensor should be far more active than the rear sensor to indicate the change in emissions. If they are both pretty much the same, the converter is malfunctioning and needs to be replaced. When an oxygen sensor malfunctions, if the computer is working properly, the check engine light on the dash should illuminate.