* Before the catalytic converter(s): This is the most common location for at least one oxygen sensor. It's upstream of the cat, measuring the un-converted exhaust gases to help the engine control system adjust the air/fuel mixture.
* After the catalytic converter(s): A second oxygen sensor might be located downstream (after) the catalytic converter. This sensor monitors the efficiency of the catalytic converter by measuring the remaining oxygen in the exhaust. It's used to help diagnose problems with the converter.
Exact location varies slightly depending on engine type (e.g., 4.3L, 5.7L). You'll need to consult a repair manual specific to your Z71's engine for the precise location. A Chilton's or Haynes manual, or even a digital version, will provide diagrams and pinpoint the location(s) based on your vehicle identification number (VIN).
Without knowing the specific engine, I cannot give you a more precise location. Visually inspecting the exhaust system near the catalytic converter(s) is your best approach, but always be careful working around hot exhaust components.
