Most modern gasoline engines use an oxygen sensor to monitor the air and fuel mixture entering your engine. Obtaining the correct air to fuel ratio is vital for getting the maximum performance and gas mileage out of your engine. With some basic mechanical skills, you can test your oxygen sensor to ensure it is functioning properly.

Preparation

Automobiles generally have anywhere from one to four oxygen sensors at various locations. Use the manufacturer's manual to determine the locations of the sensors and then visually find them. If your vehicle has multiple sensors, you will want to check all of them. Inspect all of the wires connecting to the sensors to ensure that they are not damaged and are not touching any hot exhaust pipes.

Testing

Start the car and let it idle until the engine is warm, which typically takes about 5 to 10 minutes, depending on the outdoor temperature. Letting your vehicle idle this long with raise the exhaust temperature to around 500 degrees Fahrenheit, which is necessary to provide an accurate reading. Once the vehicle has been run for 5 to 10 minutes, shut off the engine. Oxygen sensors are checked using a voltmeter and a backprobe. A correctly functioning sensor will have voltage readings that swing rapidly between 0 volts and 1 volt. Attach a backprobe to the signal wire entering the sensor, being careful not to contact any hot parts with the backprobe. The negative lead from the voltmeter should be attached to the negative battery terminal. When the vehicle is turned back on, the voltmeter should fluctuate between 0 volts and 1 volt. If the vehicle is fully warmed up and the voltmeter stays at a constant reading, the oxygen sensor is malfunctioning and needs to be replaced. If the oxygen sensor has passed the initial voltmeter test, you must still verify that the sensor will respond to a lean fuel mixture. Use the manufacturer's manual to locate the vacuum port on top of the engine and open it up, which will create a vacuum leak. When the vacuum leak is created, the voltmeter should drop to below 0.1 volt and slowly rise to above 0.5 volts when the vacuum port is closed. If the sensor does not respond to the vacuum leak, it is not functioning correctly and needs to be replaced. Repeat these two tests for every oxygen sensor on the vehicle.

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Oxygen Sensor Testing

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Oxygen Sensors