Differences Between Front and Back O2 Sensors

by Danielle DeLee

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O2 sensors, also called lambda sensors or oxygen sensors, measure the proportion of oxygen in vehicle exhaust. The sensors were first installed in vehicles in the 1970s, when the Environmental Protection Agency introduced exhaust regulations. Later, vehicle manufacturers began to install two sets of sensors: one before the catalytic converter, and one after. Physically, there's no difference between front and back O2 sensors. They function in the same way, but the vehicle's computer uses the measurements that they take for different purposes.

How an O2 Sensor Works

The first step in the invention of the O2 sensor occurred in Germany in 1899, when Walter Nernst devised the Nernst cell. At temperatures higher than 620 degrees Fahrenheit, the ceramic cell was capable of transferring oxygen ions from the gas inside of the cell to outside gases, generating an electrical current whose magnitude was dependent on the difference in the oxygen concentration of the two gases. In 1976, the Bosch company adapted the Nernst cell for use in automobiles.

Modern vehicle O2 sensors work according to the same principles that the original Nernst cells did. A zirconia bulb lined with platinum facilitates the transfer of the oxygen ions between air inside the sensor and the exhaust gases that flow over it at temperatures higher than 600 degrees Fahrenheit. Most O2 sensors have an internal heating element so that they can reach the required temperature more quickly and remain there even during idling, when exhaust temperatures can drop below the threshold for oxygen sensor function.

The Purpose of a Front O2 Sensor

Front O2 sensors analyze the exhaust coming directly from the engine. When O2 sensors were installed only in one place, they were located in the position that made them what would be called front O2 sensors today. They communicate with the engine control module, the computer that controls the mixture of fuel and air that goes into the engine. If the mixture is too rich, which means that it contains too much fuel, then the computer reduces the fuel in the mixture that goes into the engine; if the exhaust is too lean, then the computer adds more fuel to the mix. The computer tries to maintain an ideal ratio of air to fuel to minimize emissions and improve fuel economy.

The Purpose of a Back O2 Sensor

Back O2 sensors are located after the catalytic converter, a part that facilitates the conversion of pollutants in the exhaust to harmless by-products. These sensors monitor the efficiency of the converter. The computer compares the exhaust that flows into the converter with the exhaust that comes out. If the effect that the converter has on the composition of the exhaust decreases, it means that the converter is wearing out. The computer can monitor the level of the converter's operation and alert the driver when the converter needs to be replaced.

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