* Upstream O2 Sensor: Located in the exhaust manifold, before the catalytic converter. This sensor is crucial for closed-loop fuel control. It's usually closer to the engine.
* Downstream O2 Sensor: Located in the exhaust pipe, after the catalytic converter. This sensor monitors the efficiency of the catalytic converter.
Finding them: You'll need to get under the car to locate them. They are usually threaded into the exhaust system. A repair manual specific to your 2002 Chevy Impala 3.4L would be the best resource for precise location diagrams and identification. You can often find these online (often for free, or a small fee) via sites like Haynes or Chilton. A picture search online for "2002 Chevy Impala 3.4L oxygen sensor location" may also give you some visual clues. Be aware that images may vary slightly depending on options and minor manufacturing variations.
Caution: Exhaust systems get very hot. Allow the engine to cool completely before attempting to locate or access the O2 sensors.
