MIL: Malfunction Indicator Lamps

MIL: Malfunction Indicator Lamps

Illuminating the mystery of the "check engine" light
on

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.

Accessories-Only recently have some of these components been assigned control by the engine management computer. They include the fuel pump, alternator, air conditioning compressor, and radiator cooling fan.

All of these components are designed to operate with certain parameters that the computer monitors. When one of the components operates outside the assigned parameter, the computer recognizes this and illuminates the MIL. Often, the computer is also programmed to switch into "failure mode," employing a pre-mapped strategy to compensate for the failure. It may also to create a noticeable driveability symptom, so if the MIL is not enough motivation, the driver will be more inclined to take the vehicle in for service.

So, here we are at the crux (and bucks) of the matter. There's no question that when there's a driveability symptom along with a lit MIL, your vehicle should be taken to a mechanic. What if there is no driveability symptom? Engine management/emission control systems are so sensitive that the MIL will light up due to driver-induced stimuli. These might include leaving the fuel filler cap loose, driving the vehicle too hard under extreme conditions (blasting across the desert during summer with the air conditioning on max and a small trailer in tow), or using the wrong grade of fuel.

Obviously you could correct these problems yourself without a trip to a pro, and often, after several drive cycles, the MIL may reset itself. Generally speaking, manufacturers' warranties do not cover such causes, and the vehicle's owner would have to foot the bill for a diagnosis and reset of the computer. While some information doesn't come cheaply, many motorists may feel that the "better safe than sorry" adage applies here, and it's hard to argue with that. It may help to think of those MILs as good motivation to pay better attention to how you interact with your vehicle. And they sure beat the old days of idiot lights.

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