If the "check engine" (or "service engine soon") light comes on, your car's computer (On board Diagnostics, or OBD) is probably trying to alert you to a problem or fault, typically with the emissions control system. It's not immediately urgent - you can continue to drive the car - but you should get it looked at (or check it yourself) as soon as possible because at the least, your car is not running as efficiently as it should be (which means it's probably getting less than optimum gas mileage). If you drive long-term without getting whatever's not right fixed, you may cause damage to costly parts such as the catalytic converter.

The nice thing about the "check engine" light and OBD is that the car actually tells you what's wrong with it.

The not so nice thing is it's telling you in code. And you'll need an OBD code reader to access (and decipher) those codes.

Scanners are available from about $50 on the low end for a very basic model to as much as $300 or so for a professional model. The lowest cost models often only reveal the alpha-numeric code but don't translate what the code means. You need to possess (or buy) a book that lists the possible codes - and their meaning - for your particular vehicle. You're probably better off buying a model that not only reads the codes, but also tells you what the codes mean.

Using the gauge is very simple. All cars from model year 1996 and up have a standardized plug-in port (very much like the hook-ups you'll find on the back of many desktop computers) that's located in the area under the steering wheel/dashboard on the driver's side of the vehicle. It will be immediately obvious once you get down there to look for it. The gauge will work on any 1996-up OBD car or truck, irrespective of make/model.

The scanner will come with directions, but in general, you simply plug the unit's connector into the OBD port, turn the ignition switch to "run" and let the reader do its thing. It will communicate with the car's OBD computer and in a moment, whatever code triggered the "check engine" light to illuminate should be displayed. You can now fix whatever the problem was - or at least know what the problem actually is before you take the car in to be fixed by someone else. This can help protect you from being ripped-off by an unscrupulous or incompetent shop.

The scan tool will also let you turn off the "check engine" light by "clearing" the stored codes after the problem is fixed (or before). But keep in mind that unless you actually fix the problem, the system will just turn the "check engine" light back on a few miles down the road.