When you first turn the key to start your engine, the computer looks at ambient air temp (outside air temp) and engine coolant temp, as well as barometric pressure (from either MAP or MAF). Based on those parameters, it calculates whether the engine is cold or hot, whether the outside air is cold or hot and then determines how much fuel to add to get the engine started. At that point the computer is in OPEN LOOP, which means it's running off of factory programming. Once the oxygen sensors heat up to operating temperature, the computer switches to CLOSED LOOP and calculates air/fuel ratios based on what the oxygen sensor is reading. The ambient temp sensor is located in different places based on which engine you have (you didn't tell us which engine you have), but is usually in the cold air stream between the air filter and the intake manifold.