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What is a Malfunction Indicator Lamp?

By Lori Kilchermann
Updated May 23, 2024
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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, they are designed to bring a driver's attention quickly to the dashboard where the indicated problem can be monitored. Occasionally, a lamp will be used with an actual gauge and is programed to light up when there is a certain reading. On lower optioned vehicles, the light 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 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 this warning light is such that most high-performance and racing vehicles wire a light into all critical gauge systems, including oil pressure, water temperature and fuel pressure, which is designed to light up 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 coined 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 are other types of warning devices in cars.

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