* Upstream (Front) O2 Sensor: This sensor is located in the exhaust manifold, before the catalytic converter. It's closer to the engine.
* Downstream (Rear) O2 Sensor: This sensor is located in the exhaust pipe, after the catalytic converter. It's further away from the engine.
Exact location: Pinpointing the *exact* location without seeing the vehicle is difficult. The exhaust manifolds and pipes can vary slightly in design even within the same model year. You'll need to consult a repair manual specific to your 2002 Chevrolet Impala 3.9L or use a reliable online parts diagram (like those found on sites selling auto parts) using your VIN. These diagrams often show the sensor location clearly.
Visual Identification: Oxygen sensors are usually threaded into the exhaust system and have a wiring harness connected to them. They're relatively small, cylindrical components.
Caution: The exhaust system gets very hot. Let the engine cool completely before attempting to locate the oxygen sensors.
