"Uh-oh"--or another, less family-friendly phrase--is probably the first thing drivers utter when the "check engine" light flashes on. It may not mean that the engine is about to explode, but it still demands attention. Odds are, the vehicle will not pass the emissions test required for registration.

Having recently bought a new car, I was eager to try a device similar to those used by mechanics to diagnose the problems that trigger the check engine light. Offered by CarMD, a year-old firm based in Fountain Valley, Calif., the $90 tool retrieves data from the onboard computer of any vehicle sold in the U.S. after January 1, 1996. The device, which won an innovations award at the 2007 Consumer Electronics Show, will not outdo a mechanic's diagnostic equipment, but it will tell you what might be wrong in more understandable terms and how much it might cost to fix the problem. Besides, you could confidently tell the mechanic to check for a leaking exhaust manifold--even if you have no clue what a manifold is.

The driving force (ahem) behind this handy instrument is air pollution--or more specifically, the need to control it. Stiff emissions guidelines in the 1970s eventually led the Society of Automotive Engineers to devise a means of monitoring engines and their exhaust in a standard way. The culmination of that effort is the second generation of onboard diagnostics (OBD-II).

Today's automobile contains myriad sensors, explains Keith Andreasen, CarMD's director of technical services. In the mid-1990s, he says, a vehicle contained "maybe 13 to 15 sensors"; now there are "thousands in a modern-day car." Those sensors keep tabs on the emissions and relay the readings to the car's computer. For instance, if a sensor detected fumes escaping from a balky fuel gasket, it would inform the computer, which would store the data and turn on the check engine light. The computer assigns the problem a "diagnostic trouble code," such as "P0430." (Most codes start with a P, for power train; other prefix codes are B for body, C for chassis and U for the vehicle's communications network.) OBD-II employs several hundred codes, and the readers that extract this information from the computer have become indispensable to both the professional and weekend grease monkey.

Because CarMD targets the consumer, it has tried to make the diagnostic process simple. To communicate with the onboard computer, the handheld tester plugs into the vehicle's data-link connector--a rhomboid-shaped, 16-pin socket reminiscent of the parallel ports of yester decade. The hardest part about using CarMD, in fact, is finding the connector: I had to search online to learn the port's location in my 2006 Mini Cooper S. It was underneath the dashboard, to the left of the steering column, an area that forces you to twist your neck to the breaking point to get a direct view. Fortunately, my wife did not mind scrunching into the foot well with a flashlight to find the plastic flip-down cover, which had "OBD" stamped on it. CarMD has since posted the locations and images of the sockets on its Web site (www.carmd.com), although the description wrongly stated that my socket was uncovered.

After attaching the CarMD reader and turning the ignition to its second position, the unit began communicating with Darwin, the name my wife gave to our Mini. After four beeps from the reader, I yanked it out and saw that the green LED was on--everything was A-OK. I would have been extremely perturbed otherwise, considering that Darwin was only six months old. Connecting the reader to my Windows XP computer via a USB cable (no Mac OS X support), I uploaded the data to my account on the CarMD Web site. The unit correctly extracted the VIN number and description of my car.

But I needed to see how CarMD would do with the check engine light shining. According to the Aftermarket Industry Association, about 6 percent of all vehicles are cruising around with these indicators on. Given the statistics, at least one of those cars should belong to a staff member of Scientific American, which employs about 80 people in New York City. Sure enough, I shook out co-workers with car problems.