Q: Can you explain how my check engine light works? It seems the more I hear about this the more I am confused. Is it OK to drive when it's on?

A: You're not alone! This little orange light is the most perplexing component ever added to an automobile. A Check Engine light, also called a Service Engine Soon light, is officially referred to as the MIL (Malfunction Indicator Lamp) in the automotive emissions regulations specifying its use. In a nutshell, the MIL illuminates if an emissions related fault occurs. To understand the oddities in MIL function, I'll need to torture you a bit with a history lesson:

Beginning in 1980 on some vehicles and phased in sporadically on others through 1996 (light trucks 1997) an illuminated light on these vehicles indicated an open or short circuit in the engine control system's sensors or circuits, or in some cases a severe rich or lean fuel system problem. Typically the light would come on when the fault occurred and go off if the fault went away. Government mandated onboard diagnostics (now referred to as OBD-1) appeared in 1991, but there was little, if any, standardization between vehicle manufacturers regarding trouble codes, terms and diagnostic tools. OBD-I was limited to finding only glaring faults in a part of the vehicle's systems.

OBD-II changed all this in 1996/1997 requiring far more stringent testing of vehicle systems and standardization of how and when the MIL might light, trouble code definitions, component terms and more. The MIL on an OBD-II vehicle lights if any fault occurs that may cause exhaust emissions to exceed 150 percent of the mandated standard. This can be due to a faulty sensor, a wiring problem, an engine misfire, fuel vapor leakage, or degraded catalytic converter, among other possible faults. Approximately six test sequences, known as monitors, check engine operation and a variety of different component areas. Some monitors run continuously while others are at certain times/vehicle conditions, so a fault could result in an immediately illuminated MIL, or another fault may take several days or even longer to be indicated. A jiggled apart connector on the throttle sensor would be instantly flagged, while a loose gas cap may take days or longer to get your attention.

OBD-II differs also from the earlier systems in how/when the MIL may turn off. Once a fault is identified, the lamp will remain on until three successful tests are performed without incident. Depending on the monitor, this could be today or next week. A reconnected TP sensor connector might extinguish the light later in the same day, while tightening the gas cap may not produce results until the fuel tank reaches a certain level and a stubborn-to-run monitor clicks off three successful tests - maybe a week?

The most troubling aspect of the MIL is one can't be certain if a fault is of high or low importance to be fixed. If your MIL lights steadily, and the engine runs normally, try tightening the gap cap, wait a week, then make a repair appointment if it doesn't go out. If the MIL flashes and/or the engine runs rough, this needs fixing right away!

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.