* Upstream Sensor(s): These are located before the catalytic converter(s). There's typically one for each bank of cylinders (one for the driver's side and one for the passenger side). They are closer to the engine.
* Downstream Sensor(s): These are located after the catalytic converter(s). Their main purpose is to monitor the converter's efficiency. Again, there can be one for each bank. These are further down the exhaust system.
To find them: You'll need to look under your truck. The sensors are usually threaded into the exhaust pipe. They're typically cylindrical with a wire harness attached. Consult a repair manual specific to your engine size (4.3L, 4.8L, 5.3L, 5.7L, 6.0L are common for this year) to pinpoint their exact location for your particular truck. A repair manual will have diagrams to show you precisely where they are.
Caution: The exhaust system gets very hot. Allow the engine to cool completely before attempting to locate or work on the oxygen sensors.
