idiot light

An indicator light or illuminated signal, usually colored red, yellow, or orange, on a vehicle dashboard intended to convey a condition of which the driver should be aware. Most frequently, it indicates a system malfunction or a potentially dangerous condition (e.g., low oil pressure, high coolant temperature). It may be a word or words ("Charge"; "Check Engine") or a symbol which may or may not be readily identifiable. The derisive term comes from the fact that such lights typically take the place of a gauge, meter or other indicator that can provide more substantive and detailed information. The binary (on/off) nature of the light is considered inferior to an indicator which provides data on the condition, but which requires some interpretive thought by the operator.)