Idiot light

A tell-tale (or idiot light, especially in North America) is an indicator of the status or malfunction of a system within a motor vehicle. A tell-tale consists of a light bulb or an LED which lights up a symbol or text legend.

The "idiot light" terminology arises from popular frustration with automakers' use of tell-tales for crucial functions which could previously be monitored by gauges, so a troublesome condition could be detected and corrected early. Such early detection of problems with, for example, engine temperature or oil pressure or charging system operation is not possible via a tell-tale, which usually illuminates only once a fault has already occurred – thus providing no advance warnings or details of the malfunction's extent. The Hudson automobile company was the first to use lights instead of gauges for oil pressure and the voltmeter, starting in the mid-1930s.