All engines use an oxygen sensor to aid in the control of the air/fuel mixture. The ideal mixture within the engine is 14.7 parts of air to one part of fuel. If this ratio can be maintained under all conditions, emissions will be kept to an absolute minimum. The trick is to inform the control computer (ECU or engine control unit) of any change in conditions so that it can react and make necessary changes. The oxygen sensor is one of many sensors which detect changes during driving.
Located in either the exhaust manifold or the exhaust pipe ahead of the catalytic converter, the oxygen sensor reads the amount of oxygen in the exhaust flow and generates a proportional electrical voltage. This voltage is transmitted to ECU which interprets it and sends necessary messages to fuel and air control components. Remember that the oxygen sensor is reading the result of combustion and reacting to it. If there is a problem in the air/fuel mixture entering the engine, the combustion will be imperfect and the oxygen sensor will generate a signal which shows the error. The signal does not necessarily indicate that the sensor has failed, only that it has detected a different oxygen concentration.
Since the oxygen sensor is the furthest "downstream" in the combustion process, it essential to check all other sensors and controls on the engine before assuming this sensor to be bad. Obviously, if the engine is running inefficiently, replacing the oxygen sensor won't cure the problem; the new sensor will continue to correctly read the imperfect exhaust content. About the only failure common to all oxygen sensors is loose or corroded connectors in the electrical wires. If a trouble code indicates an oxygen sensor malfunction, the first place to look is at the connector, making sure the pins are clean and fit tightly together. The low voltages flowing in this system can be changed or blocked by a high resistance (poor) connection.
An accurate digital voltmeter is required for this test.
- Before testing, warm the engine to normal operating temperature. Coolant temperature must be 80-85°C (175-185°F) or more.
- Shut the engine off. Disconnect the oxygen sensor connector and connect the positive probe of the voltmeter to the sensor connector. If the connector has two terminals (California spec), use the terminal on the left.
- Ground the negative probe of the meter to the body or the engine as convenient but do not ground it back to the sensor or connect it to the second terminal.
- Place the meter where it can be seen from the driver's seat. Start the engine.
- Race the engine to about 4000 rpm and observe the meter; it should show about 1 volt (600-1000 mV).
- Shut the engine off, remove the test equipment and reconnect the sensor harness.
REMOVAL & INSTALLATION
It is more common to remove the oxygen sensor for protection or access during other repairs than to replace it because of failure. Once the sensor is removed, it must be protected from impact and/or chemical contact. Never attempt to clean the tip with solvent and never allow the tip to contact grease, oil or other chemicals. The zirconia element in the tip will be polluted and the sensor will function poorly, if at all.
- Locate the oxygen sensor. It will be located in the exhaust manifold, usually mounted either underneath or from the side.
- Follow the wiring from the sensor to the first connector and disconnect it. Do not attempt to disconnect the wiring at the sensor.
- The sensor may be obstructed by heat shields on the exhaust manifold. Remove them as necessary.
- Install the proper size wrench or socket on the flats of the sensor.
- Keeping the wrench (or socket) square to the sensor while removing it. Do not allow the wrench to become crooked or to come off the flats. Remember that the sensor has been exposed to extreme temperature and corrosive exhaust gasses. It may be difficult to remove.
- Once the sensor is removed, place it in a clean, protected location. For reinstallation, the threads of the sensor may be lightly coated with an anti-seize compound but extreme care must be taken to protect the tip and shield area of the sensor from even the slightest contamination.
- Handle the oxygen sensor carefully, protecting it from impact, and install it in place. Start the threads by hand and hand tighten it as far as possible.
- Tighten the sensor to 33 ft. lbs.
- Install the heat shields if any were removed. Tighten the bolts to 12 ft. lbs. (16 Nm).
- Connect the sensor wiring to the harness connector. Make certain the wiring is correctly run and out of the way of hot or moving components.