The upstream and downstream O2 sensors are electrically the same, however, the downstream one has more shielding to protect against water. This is necessary because H2O is a by-product of the catalytic conversion process.

The downstream O2 sensor is installed to check the efficiency of the Catalytic converter. This monitoring is required under OBD II (1990 Clean Air Act) law. If the switch rate of the downstream O2 sensor (which normally switches pretty slowly due to the consistent gases leaving the cat converter) approaches the switch rate of the upstream O2 sensor, a "catalyst efficiency monitor failure" code can be triggered. Of course, this comparison is not being made constantly; it only occurs under specific test conditions. The cat efficiency monitor is typically run at a steady light-throttle cruise with the engine warmed up.

The 96 and newer Neons do use the downstream O2 sensor input to optimize cat converter performance, however the 95 Neons do not. Chrysler has a patent on the system in which the downstream sensor is used this way.