O2 Sensor: Description: Sensor-Oxygen
WARNING: This page is about a different car, the 2008 Dodge Sprinter. However, it is still accessible from the selected car via links, so may be relevant.
The right (2) and left (4) upstream oxygen sensors are installed in the exhaust pipe upstream of the left and right catalytic converters, and downstream of the left and right exhaust manifolds.
The right (1) and left (3) downstream heated oxygen sensors are installed into the center of the left and right catalytic converters.