In order to comply with OBD II emission diagnostic requirements, an additional oxygen sensor is placed in the exhaust system behind the catalytic converter (post converter sensor). Post converter oxygen sensors are used on all General Motors vehicles that comply with OBD II. The job of the post converter oxygen sensor is to measure the amount of oxygen in the exhaust system after the catalytic converter.
This information helps determine catalytic converter efficiency and short term/long term fuel trim data values.

The only differences in the post converter sensor and the pre-converter sensor is the length of the wires, the wiring connector and protective wiring sheath. Depending on the design, there may be up to four heated oxygen sensors in the exhaust system, two before the converters and two after the converters