The Malfunction Indicator Lamp (MIL) can be a golden light of opportunity for your shop. If the light comes on, it means the Onboard Diagnostics II system (OBD II) has detected an emissions-related problem. OBD II is designed to turn on the MIL if a problem occurs that may cause emissions to exceed federal limits by 150 percent. The problem has to occur more than once, and it must be significant enough to create a potential emissions problem (one serious enough to prevent a vehicle from passing an emissions test).

In the real world, the MIL often comes on for what seems like trivial reasons (like a loose gas cap). But there's no way to know what's triggering the light until the vehicle is diagnosed. The problem may be something minor that has little or no affect on driveability, or it may be something more serious that is affecting engine performance.

The mysterious nature of the MIL, which most people call the "Check Engine" light, terrifies and confuses a lot of motorists. Except for a few luxury vehicles that actually display a fault message when the MIL comes on, most provide no information whatsoever other than indicating that something is wrong. The motorist has no way of knowing if the problem is major or minor - or what it will ultimately cost him or her to have the problem diagnosed and repaired.

Some motorists, on the other hand, seem unfazed by warning lights. As long as their vehicle continues to run...