A dashboard warning light that only illuminates when a critical fault has already occurred, such as low oil pressure or overheating. Mechanics use the term disparagingly because the light provides no real-time data or early warning trends, treating the driver as someone who cannot read a gauge. By the time the light comes on, engine damage is often imminent.
Heard on the job
The customer ignored the idiot light for fifty miles and now the engine is completely seized.