Repair 7

What Are the Front Rear Oxygen Sensors

Since the introduction of the Onboard Diagnostics II (OBD II) system in 1995, two oxygen sensors -- front and rear -- monitor engine performance and emissions in all American-sold vehicles. These small, but vitally important components communicate with the engine control unit to maximize performance and minimize emissions, and should be replaced or fixed immediately if found to be malfunctioning.

Front Oxygen Sensor Function

The front -- or upstream -- oxygen sensor, so named for its position in front or upstream of the catalytic converter in the exhaust manifold, measures the amount of unburned oxygen present in the exhaust that exits the engines combustion chamber by producing and reporting a voltage level to the ECM. This indicates the air to fuel ratio that the engine is receiving and whether or not the ECM needs to alter that mixture in order to reach the proper 14.7:1 ratio.

Rear Oxygen Sensor Function

The rear oxygen sensor is located downstream of the catalytic converter, and has a more basic function: it monitors the effectiveness of the catalytic converter. Tasked with removing many of the harmful toxins from engine exhaust, the catalytic converter is an important part of the exhaust process. The ECU compares data compiled from the rear oxygen sensor with that of the front oxygen sensor to ensure that the catalytic converter is doing its job. A non-functioning rear oxygen sensor wont affect performance, but it will likely trip a dashboard indicator.

Design

Although they have different purposes, front and rear oxygen sensors employ the same basic design to measure oxygen levels. Resembling spark plugs, these small devices produce a voltage between 0.2 and 0.9 volts, based on the amount of oxygen flowing past their bulb, and relay that information to the ECU. Most modern day oxygen sensors are electrically heated on start-up until they can maintain their own 600-degree operating temperature.