Malfunction Indicator Lamps

My, how times have changed. Back in the days when leaded fuel was sold at "service stations," as they were quaintly called, motorists had to be concerned with only a few warning lights that each monitored the condition of an important vehicle system. Colorfully dubbed "idiot lights" by the gauge-reading contingent, these did little more than indicate it was time to get your car off the road as soon as possible. Today's instrument panels, however, display a Christmas tree of warning lights that provide a wealth of information (some might say too much). Question is, what do those lights all mean? Are life, limb, and property at risk if you continue to operate your vehicle? And isn't it going to take a lot more than an idiot to interpret them all?

Warning Vs. MIL
There are two types of instrument panel displays that bear explanation here: Warning Lamps, and Malfunction Indicator Lamps (MILs). Warning Lamps generally report on a single specific component or subject, such as fluid levels (coolant, windshield washer fluid, fuel, engine oil, etc.), instrument gauges (if reading abnormally high or low) and the position of the emergency/parking brake, doors, trunk lid, and the like. These lamps are generally red in color.


The MIL, on the other hand, refers to a more complicated system monitored by a processor (computer). It usually requires testing by a service professional to determine the exact nature of the malfunction and the repair required. These systems include the antilock braking system (ABS), airbag system (SRS), automatic transmission/transaxle, traction control, and the Engine Management/Emission Control system (which we'll cover in a bit more detail). These lamps are generally amber in color, and will appear as a descriptive icon or as the initials of the system they support. Something else that they all have in common is that they will illuminate when the ignition key is first turned on, and will stay illuminated momentarily after the engine is started.


During this period, all of these computer-monitored systems are going through their programmed self-test procedures, so it's not uncommon for them to stay on for a few seconds, and then go off after passing the internal diagnosis. If any of them stay on, or come back on after momentarily going off, that indicates a problem with the system.


The "Check Engine" lamp, as it is commonly known, may actually appear using these words, or may appear as "Service Engine Soon," or a descriptive icon (such as an engine). When it is illuminated, the problem could be in one of three areas: Engine Management, Emission Control, or Accessories. Although they are distinct subsystems, the same computer controls them all.

Subsystems
Engine Management—This system is directly responsible for the fuel delivery and ignition/spark requirements of the engine, and uses a number of sensors, which input into the computer, so that it can make adjustments for optimal performance.


Emission Control—This system is closely integrated with the engine management system and reduces exhaust and evaporative emissions, while maintaining optimal driveability.