Olfactory sensor

Olfactory sensors were a type of sensors that provided droids with an artificial sense of smell. Cybot Galactica's 3PO units were equipped with such sensors.

Behind the scenes
Olfactory sensors for droids were first mentioned in the reference book Star Wars: Droid Factory. However, according to Return of the Jedi: Beware the Power of the Dark Side!, droids do not have the ability to smell, but can analyze the chemical composition of the air around them thanks to their chem-receptors.