idiot light

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Slang A light on the instrument panel of a motor vehicle that gives forewarning, as of an overheated engine.

Wiktionary

1. n. idiomatic Any warning light or indicator on the dashboard of a car, designed to alert the driver of problems, such as the parking brake being on or the oil being low.

WordNet 3.0

1. n. a colored warning light on an instrument panel (as for low oil pressure)