By David W. Gilbert

Understanding oxygen sensors

Since the early 1980s, oxygen sensors (O2S) and heated oxygen sensors (HO2S) have played a key role in the efficient operation of electronic fuel injected vehicles. In a modern vehicle, the powertrain control module (PCM) relies on information from the oxygen sensor to achieve optimum air/fuel ratio, good engine performance and control exhaust emissions. Understanding fundamentals of oxygen sensor operation, as well as new changes in technology, can help technicians quickly test and diagnose this increasingly important sensor.

Burning gasoline in the combustion chamber of an engine is a chemical reaction with fairly predictable results. Cylinder misfire, poor engine efficiency and high exhaust emissions can be the end result of too much or too little fuel in the combustion chamber. An oxygen sensor can effectively measure these combustion results. Changes in air-to-fuel ratio affect the amount of oxygen (O2) consumed during the combustion process. The best air/fuel ratio for complete combustion and emissions is a stoichiometric 14:7:1 ratio. A rich (or excessive fuel) air/fuel ratio will consume most of the oxygen during the combustion process, resulting in low exhaust oxygen content. Leaner air/fuel ratios will result in somewhat higher exhaust oxygen content. By monitoring oxygen content of the engine exhaust, the PCM can determine the ideal air/fuel ratio and adjust fuel delivery accordingly.


Oxygen sensors are typically located in the exhaust manifold and/or exhaust system. While earlier fuel injection systems used one or possibly two oxygen sensors, on-board diagnostics II (OBD-II) system emission regulations have warranted the use of multiple oxygen sensors on most vehicles. OBD-II vehicles typically have at least one oxygen sensor located ahead of the catalytic converter (upstream) and an additional sensor located just after the catalyst (downstream).

Using upstream and downstream oxygen sensors enables the PCM to measure efficiency of both engine combustion and catalyst operation.

Vehicles with dual exhaust systems may also have pre- and post-catalyst oxygen sensors for each bank of engine cylinders. The exact placement and number of oxygen sensors varies with engine configuration, vehicle design and manufacturer.

One of the most common types of oxygen sensors is the zirconium dioxide oxygen sensor. The O2 sensing component uses a solid-state electrolyte made up of a zirconic ceramic material that acts like a galvanic battery electrolyte under certain conditions. When the sensing element is cold, the zirconia material behaves similar to an insulator. At elevated temperatures, the zirconia material performs more like a semiconductor, and can generate a characteristic voltage output on the sensor connections.

In construction of the zirconia sensing element, a porous platinum electrode material covers the inner and outer surfaces of the zirconia solid-state electrolyte. The inner surface of the sensing element is exposed to an outside air reference, while hot gases in the exhaust stream surround the sensor's outer portion. Oxygen content of outside air is approximately 21 percent, while exhaust gases have much lower oxygen content - between 1 percent and 3 percent.

Differences in the two oxygen levels, and the electrolytic properties existing between the two platinum electrodes, allow ion transfer to take place and generate a small electrical charge. Oxygen ions are electrically charged particles that flow through the zirconia sensing element when there is a disparity in oxygen levels. The greater the ion flow, the higher the voltage produced. Once the zirconia sensor element reaches an operating temperature of 572 degrees Fahrenheit to 680 degrees Fahrenheit, signal voltage output can range from near zero to 1 volt - depending on the oxygen content of the exhaust gases.

Basically, the zirconium O2 sensor compares the oxygen content of exhaust gases with oxygen from outside air. Voltage produced by the O2 sensor depends on the amount of oxygen in the exhaust. If exhaust oxygen content is low, such as a rich air/fuel ratio, the voltage output from the sensor may be as high as 1 volt. A lean air/fuel ratio increases the exhaust oxygen content, resulting in a low voltage from the sensor.

In normal operation, O2 signal voltage is routinely varying from almost zero to 1 volt. An O2 sensor signal voltage above approximately 0.45 volts is recognized by the PCM as a rich exhaust; below 0.45 volts as a lean exhaust. The goal of the PCM is to keep O2 voltage moving across the 0.45 volt rich/lean switch point for optimum fuel efficiency and emissions.

The PCM will set an O2 sensor diagnostic code if the sensor does not produce a voltage signal, stays rich too long, stays lean too long, does not switch rich/lean (center too long), or does not switch rich/lean fast enough. OBD-II vehicles may also run PCM diagnostic tests called monitors, which compare and analyze sensor readings to verify proper component operation.

Since OBD-II vehicles may have multiple oxygen sensors located some distance from the engine exhaust ports, these sensors are generally heated to speed the warm-up time period. The HO2S incorporates an internal electric heating element to bring the O2 sensor up to operating temperature quickly (under 35 seconds). Internal heating elements usually operate continuously while the engine is running to maintain an operating temperature of approximately 1292 degrees Fahrenheit to 1472 degrees Fahrenheit. Heated O2 sensors operate at a more consistent temperature and allow greater flexibility of placement locations in the exhaust system.


There are three common methods of controlling the heating element in oxygen sensors. The first method provides a power source to the heater from the ignition switch or a relay anytime the ignition is turned to the run position. This method was used on many pre-OBD-II vehicles without heater diagnostics. A second method supplies power to the heater through a PCM controlled relay. By controlling the heater power relay with the PCM, the circuit can be checked during key-off/engine-off periods. The third method is limited to newer vehicles equipped with Fast Light Off (FLO) oxygen sensors. These sensors have a larger heater for quick sensor warm-up and are current flow limited through the PCM. Note that due to heater design and current draw differences, FLO oxygen sensors cannot be interchanged with other types. Inside the PCM is a switching transistor that pulse-width modulates the power supply, thus controlling current flow in the heater circuit. Using this type of PCM control, the FLO oxygen sensors can reach full operating temperature in as little as five seconds after startup.

One aspect of OBD-II vehicle diagnostics is the ability of the PCM to periodically test the HO2S for possible heater failure. As the name implies, the HO2S heater monitor (or test) is used to check the operation of the internal heater. Because the O2 sensor may be warmed by exhaust with the engine operating, a PCM actuated heater monitor typically runs after a predetermined ignition key-off/engine-off period. Specific enabling factors for this monitor may vary between manufacturers. When the heater monitor is running, the PCM measures the internal resistance of the sensor element as it heats up. Remember, the zirconia material changes conductivity with temperature. By energizing the HO2S heater element, and simultaneously monitoring the sensor signal circuit, the PCM should see the internal resistance of the sensor signal circuit go down as the temperature increases. This monitor fundamentally checks the integrity of the O2 heater element and its related circuits, as well as the O2 sensor signal circuit.

Zirconia oxygen sensors can have one, two, three or four wires depending on the vehicle application. One or two wire O2 sensors are not electrically heated and will have a signal wire and possibly a ground wire. Heated O2 sensors usually have three or four wires: two wires for the heating element, one signal wire and possibly a ground wire. An electrical wiring schematic can be helpful to positively identify connector pin locations and wire colors. Oxygen sensors that are not equipped with a ground wire must have a well-grounded exhaust system to complete the sensing circuit. Basic electrical wiring circuit checks should be made to determine if the vehicle's wiring harness has good continuity and is free from short circuits.

Testing and diagnosis of the O2 sensor heater and circuit is a relatively simple task. Most heaters are a positive temperature coefficient element, meaning the resistance will go up as the element heats up. In normal operation, the increased resistance of a hot sensor will naturally limit current flow in the circuit. Use a digital multimeter to check the sensor heater element for continuity. Exact heater resistance specifications may vary depending on the vehicle and sensor location. Heater element resistance should be about 4 to 7 ohms for a sensor at ambient temperature. Expect somewhat lower resistance values for FLO type sensors.

Another fundamental diagnostic test is checking the vehicle's wiring harness for power and ground to the O2 sensor heater. Take into account that PCM-controlled heater circuits may require the engine running before the circuit will be powered up. Connecting a 12-volt test light between the power supply and ground can determine if the heater circuit is operational. Some technicians prefer to check heater circuit current flow using an ammeter. To measure current flow, connect a digital multimeter in series between the vehicle wiring harness and the sensor heater. This testing method ensures proper heater and circuit performance throughout the temperature range of the sensor. Amperage values can be from approximately 1.5 amps at ambient temperature to 200 milliamps at full operating temperature.

If engine performance checks or scan tool data suggest a potential O2 sensor malfunction, don't forget the basics. Always conduct a visual inspection of the sensor, electrical connections and wires. Electrical connections should be clean and tight. Be sure the wiring harness is routed away from high voltage ignition components and hot exhaust pipes. Inspect the exhaust system for leaks or holes that may affect the O2 sensor readings. The O2 sensor measures oxygen, so a tightly sealed exhaust system is very important for accurate sensor readings. Determining if the O2 sensor is truly defective or another electronic engine control component has failed is perhaps the most difficult obstacle for an accurate diagnosis. Careful review of scan tool information and sensor readings can usually identify any other problems or conditions.


David W. Gilbert is an Associate Professor of Automotive Technology at Southern Illinois University Carbondale. He holds a Master of Science degree from Oklahoma State University and is also ASE certified as a master automotive technician, master engine machinist and advanced engine performance specialist (L1).


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