Oxygen Sensors & Components

An oxygen sensor detects the amount of unburned oxygen in the engine exhaust and references it to the amount of oxygen in the atmosphere. It creates a voltage signal based on the difference that is sent to the engine control computer. The amount of voltage indicates whether the air/fuel mixture is rich or lean. In response, the computer varies fuel injector pulse width (on time) to control the air/fuel mixture. Although the computer also relies on inputs from other sensors regarding engine temperature, engine load, and throttle position, the oxygen sensor is the one that provides feedback and allows the engine control system to operate in “closed loop”, the operating mode that provides optimal catalytic converter efficiency and minimal exhaust emissions. When an oxygen sensor gets old or is damaged by contaminants, it may not be able to provide a timely signal to the computer. Without accurate feedback the computer can’t adjust the air/fuel mixture to suit engine operating conditions, which may result in increased emissions. If you’re in need of a replacement oxygen sensor, you’ll find what you need to restore performance and emissions compliance here on our digital shelves.

An oxygen sensor has a zirconium dioxide sensing element coated on both sides with platinum, which also serves as electrodes, encased in ceramic. The electrodes are connected to wire leads that attach to the vehicle wiring harness and the ceramic enclosure is encased in a steel shell. The steel shell is threaded for installation in the exhaust and it protects the sensing element. Openings in the steel shell adjacent to the sensing element tip allow the sensor to be exposed to exhaust gas. All modern oxygen sensors contain a heating element that is also connected to the wiring harness. Openings at the top of the sensor allow outside air to enter the sensor, which is then heated. When heated, oxygen ions move from the inside to the outside platinum layer, and the potential difference generates a voltage. When the air/fuel mixture is rich and there is little unburned oxygen in the exhaust, up to 0.9V is generated. When the mixture is lean and there is a high concentration of unburned oxygen in the exhaust, the voltage signal will fall to 0.1V.

All late model cars and trucks have at least 2 oxygen sensors, one mounted in the exhaust manifold or exhaust pipe before the catalytic converter and one mounted in the exhaust after the converter. Vehicles with V6 or V8 engines have at least 4, one before and one after each converter in the exhaust from each cylinder bank. The upstream, before catalyst, oxygen sensor is the primary sensor that provides feedback to the engine control computer. High voltage indicates a rich air/fuel mixture and low voltage indicates a lean air/fuel mixture. When the voltage signal is low the computer increases the injector pulse width to richen the mixture; when the voltage signal is high, the computer reduces the injector pulse width to lean the mixture. The voltage signal from the oxygen sensor constantly cycles between rich and lean conditions, as often as several times per second, and in turn, the computer constantly adjusts the air/fuel mixture to achieve a balance that allows the catalytic converter to function effectively.

Oxygen sensors operate at approximately 600°F (315°C), and the heater is included in modern sensors to allow them to heat up faster so the system can quickly go into closed loop. When viewed on a diagnostic oscilloscope, an upstream oxygen sensor at operating temperature should produce a uniform waveform that varies between 0.1V and 0.9V with a midpoint of approximately 0.45V. The downstream oxygen sensor is also known as a catalyst monitor because it indicates converter efficiency. In contrast to the upstream sensor, the downstream sensor should have a relatively flat waveform at approximately 0.45 volts if the catalytic converter is functioning properly. If the downstream sensor signal and diagnostic scope waveform are similar to the upstream sensor, it means that the converter is not functioning efficiently. If this occurs a Diagnostic Trouble Code (DTC) will be set in the computer and the CHECK ENGINE light will illuminate.

Oxygen sensors deteriorate with age and contamination. Sensors can become contaminated due to exposure to silicates from coolant leaks, silicone from using the wrong type of RTV sealant, phosphorus from oil consumption due to worn piston rings and/or valve guides, excessive carbon buildup, and exposure to other chemicals, even those in gas additives. When the sensor becomes contaminated it loses the capability to quickly react to changes in the air/fuel mixture, resulting in reduced converter efficiency and increased emissions. If the sensor fails completely, the system will default to a fixed rich air/fuel mixture, which increases emissions and reduces fuel mileage. A deteriorating or completely failed oxygen sensor will set one or more DTCs and illuminate the CHECK ENGINE light. Oxygen sensor function can be checked with a scan tool or digital multi-meter, or as indicated above, with a diagnostic oscilloscope. Sensor response can be checked by artificially enriching the air/fuel mixture with propane and by removing a vacuum line to create a lean condition.

Instead of the conventional oxygen sensor described above, some newer vehicles use an air/fuel ratio sensor. Whereas the traditional sensor is a narrow range sensor that provides an alternating rich/lean signal to the engine control computer, an air/fuel ratio sensor can measure a much broader range of air/fuel mixtures, which is why such sensors are called “wideband”. The signal from an air/fuel ratio sensor is a much more accurate representation of the actual air/fuel ratio. Instead of the sensor generating a voltage, the engine computer provides a reference voltage to the sensor, and the computer then monitors the sensor output current. Although an air/fuel ratio sensor functions differently than a conventional oxygen sensor, it can suffer from the same defects and exhibit the same failure symptoms.

No matter what vehicle you’re working on or the type or position of the defective oxygen sensor, we have the replacement sensor needed to restore proper performance, reliability, fuel economy, and emissions compliance. We offer components that are manufactured to original equipment specifications, so when repairs are completed you can count on the performance your vehicle was designed to deliver. In addition to oxygen sensors, we have related components that may be needed to affect a complete repair including connectors, relays, wiring harnesses, extension harnesses, bungs, nuts, plugs, gaskets, and more.