A malfunction indicator lamp is used in the dashboard of a vehicle to alert drivers to a potential problem. Commonly referred to as idiot lights, the malfunction indicator lamp is designed to bring a driver's attention quickly to the dashboard where the indicated problem can be monitored. Occasionally, a malfunction indicator lamp will be used with an actual gauge. In this application, the malfunction indicator lamp is programed to illuminate when a certain reading on the gauge is reached. On lower optioned vehicles, the malfunction indicator lamp is primarily the only trouble indicator available to the driver.

Commonly wired to an electric sensor on the vehicle's engine or chassis, the malfunction indicator lamp is triggered when the contacts inside of the sensor are forced together to create a closed circuit. In the case of a temperature sensor and indicator lamp, when the temperature of a vehicle's engine reaches an unsafe level, the mechanism inside of the sensor is designed to snap closed. This sends an electrical current to the malfunction indicator lamp, causing it to light up. When the temperature drops below the danger level, the mechanism inside of the sensor releases, cutting the electrical current and extinguishing the light.

The effectiveness of the malfunction indicator lamp is such that most high-performance and racing vehicles wire a light into all critical gauge systems. Oil pressure, water temperature and fuel pressure all typically receive a warning indicator lamp designed to illuminate when a dangerous operational level is reached. This will alert the driver to a potential problem and allow more focused attention on that particular gauge reading.

The term "idiot light" was given to the malfunction indicator lamp because people were not always able to decipher the information received from a gauge. Engineers reasoned that even an idiot could realize there was a problem if a bright red light suddenly lit up on the vehicle's dashboard. Regardless of the offensiveness of the moniker, it proved true and the name stuck. The lights work so well that there has never been an alternative to the warning device, and it is used on nearly every model of every vehicle manufactured in the world. In addition to the malfunction indicator lamp, air bag systems, anti-lock brake warnings and seat belt warning indicators present other types of warning devices in cars.