Where are all the o2 sensors on 99 Chevy silverado with 5.3?

A 1999 Chevy Silverado 5.3L will typically have one O2 sensor per exhaust manifold, for a total of two O2 sensors. They're located before and after the catalytic converter on each exhaust manifold.

* Upstream (Pre-Cat): One sensor is located on each exhaust manifold *before* the catalytic converter. These sensors measure the oxygen content in the exhaust before the catalytic converter to help the engine's computer adjust the air/fuel mixture.

* Downstream (Post-Cat): The other sensor is located on each exhaust manifold *after* the catalytic converter. These primarily monitor the catalytic converter's efficiency. They're less critical for fuel mixture adjustments than the upstream sensors.

Important Note: While the typical setup is two, there's a slight chance a modification or unusual configuration might exist. If you need to be absolutely certain of their location before working on your truck, consult a repair manual specific to your vehicle's year and engine. A visual inspection is always recommended.