* Upstream (pre-catalyst) sensor: Located in the exhaust manifold, before the catalytic converter. There's usually one on each exhaust pipe (if it's a V6). Finding the exact location requires looking under the car, near the engine, following the exhaust pipes.
* Downstream (post-catalyst) sensor: Located in the exhaust pipe, after the catalytic converter. Again, there's usually one on each exhaust pipe (if it's a V6). This one will be further down the exhaust system than the upstream sensor.
To be absolutely certain: Consult your vehicle's repair manual or a reliable online automotive parts database (like AutoZone, Advance Auto Parts, or RockAuto) using your VIN. These resources will provide diagrams showing the precise location of the sensors for your specific Impala model. The engine (3.4L V6, 3.8L V6, or 5.3L V8) will influence the exact placement.
