How to Troubleshoot an Oxygen Sensor

by Dan Ferrell

The oxygen sensor in your car is constantly monitoring oxygen content in exhaust gases from your engine. This helps the vehicle computer adjust for the proper fuel/air mixture, which affects drivability. If you are looking for the cause of an engine-performance problem, a sudden increase in gasoline consumption, or if the car computer code is pointing you toward the oxygen sensor, follow these simple steps to verify the oxygen sensor's proper operation. Even performing these tests as part of a maintenance program will greatly benefit your car engine. So let’s get started.

Step 1

Locate the oxygen sensor in your vehicle. Follow the exhaust pipe at the exhaust manifold. You should find the sensor threaded into the exhaust pipe ahead of the catalytic converter.

Step 2

Inspect the oxygen sensor electrical connector for damage. Make sure it is free of dirt and grease. Keep the sensor harness a reasonable distance from the exhaust to avoid damage to the connector and wires.

Step 3

Idle the engine for about three minutes, then turn off the engine. Remove the oxygen sensor using a ratchet and oxygen sensor removal socket, keeping your hands and arms away from the exhaust pipe to avoid burning yourself. Closely inspect the tip of the sensor and make sure it is not heavily coated with carbon deposits. Thread the sensor back into the exhaust pipe.

Step 4

Check the voltage signal coming out of the oxygen sensor using a 10-megaohm digital voltmeter. Hook up the voltmeter's red probe to the sensor’s signal wire and the black probe to engine ground.

Step 5

Start the engine and let it idle for about two minutes while watching the digital reading on the voltmeter. It should be fixed for a short period at around 0.1 or 0.2 volts. After two or three minutes, the voltage reading will begin to fluctuate between 0.1 and 0.9 volts. If the sensor takes four minutes or more to begin fluctuating, replace it.

Step 6

Jot down in a notepad the lowest and highest voltage reading from the digital multimeter in any one-minute period. The voltage should fluctuate constantly between 0.1 and 0.9 volts. If the voltage goes above this range, remains below 0.5 volts or stays fixed at a particular voltage, replace the oxygen sensor.

Step 7

Disconnect the PCV valve vacuum line with the engine running. As the engine stumbles, watch the voltmeter reading. It should go down to about 0.2 volts. Reconnect the vacuum line.

Step 8

Open and close the engine throttle with a quick motion. The sensor’s output voltage should go up and down accordingly.

Step 9

Block the air intake duct using two or more rags. The sensor’s output voltage should go up to about 0.9 volts. If the sensor response is out of these voltage specifications, replace it.

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