How oxygen sensors work
O2 sensors are also known as oxygen sensors or lambda sensors. Where are O2 sensors located? They're positioned within a vehicle's exhaust system and are used to check that the air/fuel mixture being fed into the engine is correct. They also check that the catalytic converter is operating efficiently.
Upstream oxygen sensors generate a small variable voltage signal in proportion to the difference between the oxygen content in the exhaust stream and the oxygen content in the ambient air. The Powertrain Control Module (PCM), which is the brain of the emissions system, uses this information to maintain the proper air/fuel ratio.
Downstream oxygen sensors (sometimes called ‘catalyst monitor’ sensors) are so-named because they are ‘downstream’ from the catalytic converter in the exhaust system. Downstream sensors also generate a small variable voltage signal to let the PCM know if the catalytic converter is operating at maximum efficiency. If not, an error code is set and the MIL (more commonly known as a ‘Check Engine’ light) is illuminated.
Typically there are multiple oxygen sensors - two for each cylinder bank. Four-cylinder engines have two sensors, while V6 and V8 engines have four sensors. The upstream oxygen sensor(s) are located below the exhaust manifold flanges and above the catalytic converter. The downstream oxygen sensor(s) are located in the exhaust pipe, behind the catalytic converter.