Maintenance Light

All right ... it's great that you can turn the indicator off. But what's bugging me is the common perception as to what that light means. That light doesn't come on to let you know that your engine is malfunctioning. It really has nothing to do with that at all. (Although as a side effect of it's purpose, it can inform you of problems). Rather, the MIL (malfunction indicator light) is designed to alert you of problems with the OBD II system (on board diagnostics) which exist only to monitor emissions! That light comes on when there is a leak in fuel pressure (gas vapor escaping from the gas cap or tank), if the cylinders are misfiring (wasted fuel or incorrect burn) if the exhaust has a leak (catalytic converter not being used), catalytic converter failures (O2 sensors detecting proper scrubbing not taking place) or the O2 sensors fail (not heating up and not detecting O2). There are a variety of other things detected as faults - but the MIL light lets you know when something to do with your emissions has triggered "an event." Many of those events are completely innocuous. Some of them aren't. But the light comes on when a fault occurs.

BTW - when your vehicle is inspected (depending on your state), most of the emissions tests for modern vehicles are performed by plugging their equipment into the diagnostic OBD II port - NOT measuring actual emissions. That equipment can tell whether the problem was fixed or the code was simply cleared. If the light comes back repeatedly, they can also tell that you were just clearing it and not servicing the fault.

So ... that said, just because you can clear the light, doesn't mean you should. It came on for a reason. I plugged my laptop into a co-workers car and read multiple faults from his engine including bad O2 sensors and engine knock. Stuff like that will keep coming back and he now knows exactly what to ask his mechanic to fix.

Besides, if you're into the "green chic", clearing your emissions fault indicator without reading the fault code is like a Catholic spitting in the Holy Water.

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