A small piece of electronic equipment regulates the air to fuel mixture, fuel economy and the toxicity of the emissions in your Honda Civic. The oxygen sensor is mounted inside the exhaust system of the vehicle and measures the amount of oxygen in the emissions. The information is sent to the electronic control unit where adjustments are made to the operation of the engine. A bad O2 sensor will negatively effect the performance of your vehicle. Perform these simple tests to determine the integrity of your oxygen sensor.
Park the vehicle in an area away from traffic, turn off the engine and allow it to cool. Prop open the hood and locate the upper oxygen sensor. It is mounted on the exhaust manifold in the rear of the engine compartment behind the engine block. It will be a small plug and wiring harness sticking out of the exhaust crossover pipe.
Spray the oxygen sensor with lubricant compound for ease in removal. Apply the O2 sensor removal wrench and thread the wires through the socket channel on the wrench. Turn counterclockwise to remove the O2 sensor from the exhaust manifold.
Use your fingers to unfasten the wiring harness from the O2 sensor's wires. The harness is plastic and the clip will break with too much force.
Inspect the head of the sensor. A gray or reddish color indicates normal wear and tear on the sensor. A dark black color indicates the sensor is letting the fuel run rich and must be replaced.
Attach the positive lead on the voltmeter to the oxygen sensor wire and set the voltmeter to the two volt setting. If your oxygen sensor has more than one wire, attach the positive lead to the black wire on the sensor. Place the negative lead of the voltmeter to the body of the sensor and hold it there with your thumb.
Light a propane torch and carefully insert the head of the oxygen sensor into the flame. Wear protective gloves and safety glasses for this. The voltmeter reading should rise to 0.9 volts within one minute of being in the flame. When removed the voltmeter reading should descend below 0.1 volts. If it does not yield these readings, the sensor is faulty and must be replaced
Evaluate the other oxygen sensor. A Honda Civic has two oxygen sensors: one on the exhaust manifold and the other on the exhaust pipe near the muffler. Chock the front wheels, raise the rear end of the vehicle with two car jacks and support the frame on jack stands. Slide under the vehicle to access the second oxygen sensor. Remove and test it following the procedure described above.
