It glows on your car's dashboard and instantly ignites terror and loathing in the hearts and wallets of drivers; the dreaded "Check Engine" warning light.

For some, it is a minor irritant to be ignored. Even Public Television's "Car Guys" joke that the remedy is to put a piece of duct-tape over it. Some cynics believe the "Check Engine" light was invented to get vehicle owners to schedule more visits to the dealer for service. Some cars, after all, seem to have such a sensitive "Check Engine" light, and driving over a stick of gum is enough to flick it on.

Before making any assumptions about what to do, though, or whether a call to the dealer's service department should be an automatic response, it is a good idea to understand the light and what makes it come on.

What is it?

The check engine light is usually a warning lamp on the instrument panel of your car that lights up when the car's engine computer detects a problem. All cars have some sort of on-board diagnostic system, though some higher-end models may supplement the simple light
with a more detailed graphic display to provide additional information about what's wrong.

How does it work?

When the check engine light flashes briefly, it means the car has experienced a momentary problem. These hiccups happen sometimes, but aren't much cause for alarm. If the check engine light comes on and stays on, however, that's an indication of a problem that could be
serious. If the check engine light blinks constantly, you need to stop your car immediately, as it has experienced a major malfunction and continuing to drive the car could cause serious and wallet-wilting damage to the engine or even lead to a dangerous situation.

But the check engine light in modern cars is more than just an "idiot light" as it was once called. Since 1996, all cars sold in the United States have been required to have something called OBD-II in them. "OBD" stands for On-Board Diagnostics, and it's a standardized way for the engine control computers to report problems. When the check engine light comes on, the system records a code that corresponds to the problem the engine computer experienced. If you look under your dashboard on the driver's side, there's a funny little connector that looks like it belongs to a video game system. That's the OBD-II port, and if you plug in the proper equipment, the computer in the car can tell you what went wrong.

The neat thing about OBD-II is that it's ubiquitous. While some manufacturers have equipped their cars with other proprietary diagnostic ports for their specific dealership service technicians, OBD-II is the same on every car. This means that it's easy and cheap for mechanics and do-it-yourselfers to buy what's called a "scan tool" or use a laptop and some specialized software to connect to the car and download the fault code from the OBD-II system. CarMD is one such service that consumers can buy into and use to diagnose their own vehicles.

Why would I want it?

You certainly don't want your check engine light to come on. But if it does, downloading the fault code from the OBD-II port to one of the after-market program on your computer, such as CarMD, can tell you what the computer thinks went wrong with your car. Sometimes these errors are just simple things: Often the check engine light will illuminate when the gas cap has not been tightened. But other times, serious errors will be reported, like problems with the catalytic converter or conditions that cause the car to run rough or lose power. While these problems will all make your check engine light glow like a Christmas tree, the big advantage to OBD-II is that once you know what the computer in your car thinks went wrong, it can make it much easier to diagnose and repair the problem. That means your mechanic takes less time to make the repair, and you spend less paying the mechanic.

Is there any downside?

There is one big criticism of the check engine light system, which is that when it comes on, it doesn't really tell the driver enough. Since the range of problems that can cause the light to activate is so broad, drivers don't know what to do when they see the light.

Is it a small problem, like the gas cap not being tightened? If that's the case, than a trip to dealer or mechanic can be a huge waste of time and money. But if the issue is more serious and gets ignored, there is a risk of damage to the car. If a dealer sees that the check
engine light was on, but the owner kept driving the car, that can become grounds to void the warranty on the damaged parts. If ther car is still under warranty, the best idea is to just take it to the dealer. Anything wrong should be covered.

What vehicles offer it?

Every vehicle sold new today has an OBD-II port and some sort of illuminated check-engine light.

Bottom line

Understanding what t means when your check engine light comes on, and how to best respond to it can help you keep your car running well, creating less pollution and saving you money.