O2 Confusion 2000-2002 NISSAN SENTRA VEHICLES“OXYGEN SENSOR CONFUSION” These vehicles utilize two separate oxygen sensor configurations. Federal and California vehicles utilize 4 oxygen sensors: two are located upstream in the exhaust manifold and the second two downstream after the pre-catalytic converters. The first upstream sensor monitors cylinders #1 and #4. While the second upstream sensor monitors cylinders #2 and #3. There is another engine option designated as “CA” (Clean Air) which utilizes one upstream 6 wire air fuel sensor and one regular downstream O2 sensor. “CA” vehicles were always confused with California emissions vehicles. A quick check on how many O2 sensors that the vehicle uses can easily help identify whether or not the vehicle is a Clean Air vehicle. Visit our Resource Library for available downloads