On a 1989 Chevy Astro van, "EST" doesn't refer to a specific component or system. There's no standard automotive acronym "EST" related to the Astro's functionality. It's possible you're misremembering a label, abbreviation, or perhaps seeing a sticker or marking that's not directly related to a system's function.
Could you be thinking of something else? Perhaps a specific part number, a code on a gauge, or a word from an instruction manual? Providing more context would help me understand what you're asking about.
