idiot light

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A light on the instrument panel of a motor vehicle that gives forewarning, as of an overheated engine.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a colored warning light on an instrument panel (as for low oil pressure)