Oxygen Sensor Testing

The oxygen sensor monitors the oxygen content of the exhaust and relays this information to the electronic control unit (ECU). The ECU can then make changes in the fuel mixture to improve exhaust emissions and drivability.

The sensor produces a small voltage based on the oxygen content in the exhaust relative to the outside air. When the mixture is lean the oxygen content is high and the voltage is low. When the mixture is rich, the voltage is higher.

The voltage can be measured with a voltmeter capable of measuring with a minimum of 10 mega ohms impedance. Do not use a meter with less, as it may damage the ECU.

Connect the positive test lead to the oxygen sensor and the negative lead to ground. Start the car and warm it up to operating temperature (the oxygen sensor does not work until the temperature reaches 600 degrees).

Note: Leave the oxygen sensor connected, and probe through the rear of the connector when connecting the volt meter.

At idle the oxygen sensor should be between .04 and 1.0 volts. The voltage should fluctuate. This indicates that the sensor is working. If it’s steady or there’s no voltage, the sensor is probably bad. Another, less accurate, test is to simply disconnect the sensor when the car is running; if the idle fluctuates after about 30 seconds, plug it in again and the engine should fluctuate again until it smoothes out the idle. If no change occurs, then the sensor has probably failed.