The fuel tank pressure sensor on a 1999 Chevy S10 4-cylinder is
located inside the fuel tank. It's not something you can easily access without dropping the fuel tank. There isn't a single, universally accessible location for it; its precise placement within the tank will vary slightly depending on the specific fuel tank and model year variations.
You'll need to consult a repair manual specific to your 1999 Chevy S10 4-cylinder for a diagram showing the exact location within the tank. These manuals often include exploded diagrams of the fuel tank assembly. Trying to locate it without a diagram is extremely difficult and potentially dangerous due to fuel system components.
