The fuses for the HVAC (heating, ventilation, and air conditioning) system in a 2003 Chevy Impala are
located in the underhood fuse block. There isn't a single fuse for "fans," but rather multiple fuses and possibly relays related to the blower motor, AC compressor, and other components. You'll need to consult your owner's manual for the specific fuse numbers and their corresponding functions. The manual will have a diagram of the fuse block showing the location of each fuse.
Important Note: Before working with any electrical components, always disconnect the negative battery terminal to prevent accidental short circuits.