Identify the sensor by exhaust position before buying. The two sensors under the hood where the exhaust manifold meets the downpipe are the front and middle upstream sensors. The third sensor, farther back on the exhaust, is the rear sensor. Match the replacement to that location.
The owner of a 2000 Mazda Millenia 2.5L asked where the catalytic converter is and whether the oxygen sensor against the firewall on the left side is upstream or downstream. A Mazda tech said the vehicle has three sensors and confirmed the engine. Another expert explained that the front and middle sensors near the manifold and downpipe are upstream, while the third is farther back on the exhaust, and referred to diagrams labeling them front, middle, and rear.
