Oxygen (O2) sensors are part of the engine management system and are used to perform two important functions. The “upstream” O2 sensor(s) in the exhaust manifold(s) monitor the air/fuel mixture, while the “downstream” O2 sensor(s) to monitor the catalytic converter(s).
Most O2 sensors are the switching type that generate a rich or lean voltage signal depending on how much unburned oxygen is in the exhaust. A few, like the “titania” O2 sensors in some older Jeep, Nissan and Toyota applications change resistance to indicate a rich or lean condition in the exhaust. In many late-model applications, an “Air/Fuel” sensor (which is actually a “wide band” O2 sensor) is used to monitor the fuel mixture.
On the fuel management side, the engine computer (Powertrain Control Module or PCM) uses the input from the upstream O2 sensor(s) to readjust the fuel mixture as needed for optimum emissions, fuel economy and performance. Input from the downstream O2 sensor(s) is used to detect any problems with the converter(s), and to fine-tune fuel trim.
If a fault occurs in an O2 sensor or its internal heater or wiring circuit, the OBD II system should detect the problem, set one or more fault codes and turn on the Check Engine light. If diagnosis confirms a bad sensor (and not a wiring fault or some other type of engine problem that is affecting the sensor reading), the sensor needs to be replaced.
If one upstream O2 sensor in a high-mileage vehicle with a V6 or V8 engine has failed, chances are the other upstream O2 sensor on the opposite cylinder bank is probably reaching the end of its service life too. Replacing both sensors is recommended to restore like-new performance and fuel economy.
Downstream O2 sensors tend to be longer-lived because they are behind the converter and exposed to less heat. On the other hand, their undercar location exposes them to more road splash and corrosion. Because of this, a new downstream O2 sensor should be installed if the catalytic converter is being replaced.
Replacement oxygen sensors are available in two versions: exact fit and universal. Exact fit O2 sensors come complete with a wiring harness and connector that plugs into the engine wiring harness the same as the original sensor. Exact fit are fast and easy to install because they do not any require cutting and splicing of wires. Universal O2 sensors, on the other hand, do require some additional installation effort but allow fewer part numbers to provide much broader application coverage.
Related sales opportunities for customers who are replacing an O2 sensor include a special oxygen sensor removal socket with a cutout for the sensor wire, antiseize compound for the sensor threads (if not preapplied to the sensor), and crimping pliers for joining wires on a universal O2 sensor.