"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.

For our online project manager David Yu and his 1999 Acura CL, the tester produced trouble code P0401: "EGR System Insufficient Flow Detected." Among the possible causes, as the CarMD Web site describes them: the EGR valve source vacuum supply line was open or restricted; the EGR exhaust manifold passages were clogged or restricted; or the EGR valve assembly or solenoid valve was damaged or had failed. The most likely fix, according to CarMD, was replacing the EGR valve and cleaning the ports. Making sense of the arcane lingo, some of which is gibberish to the casual driver and grammarian alike, might need some Web research. But for most people, knowing that EGR stands for "exhaust gas recirculation" is much less important than knowing the impact on their wallet. Factoring in geographically weighted labor costs, the program estimated that David should expect to pay $487.70 for repairs.

Mike Florek, our general manager, had a more elusive problem with his 2003 Nissan Pathfinder, which had been flashing its check engine light for some months. CarMD implicated the catalytic converter or the heated-oxygen sensors. When we ran the test in December, CarMD did not offer any suggested fixes--it reported that the problem appeared too infrequently. A few weeks later, though, the details of the repair and its cost appeared (technicians keep updating CarMD's database). The fix: a replacement catalytic converter. The bad news: $828.96. A skilled mechanic, though, would still need to inspect the car thoroughly, because whatever damaged the converter in the first place could still be around to wreck the replacement. CarMD bills the device as a means to "empower" drivers so that they will not, for instance, feel intimidated by sketchy repair people.

But CarMD can also pay for itself, as Silvia De Santis, our prepress and quality manager, can attest. No one likes to see the check engine light come on, but it's really inconvenient when it happens while you are in the throes of labor. (To her husband's credit, he continued driving to the hospital.) Within days after the birth of their son, they brought their Toyota to a mechanic. "After $200 of diagnostic tests," Silvia says, "they told us it was probably an improperly closed gas tank cap"--the most common reason for the light to turn on. I experimented with removing my Mini's gas cap; sure enough, after the check engine light came on, CarMD advised me to "check for loose gas cap and replace if necessary."

The idea for the company began in 1997, when Andreasen and Ieon C. Chen, both working for a developer of automotive test equipment, discussed ways to market a consumer-friendly method of interpreting OBD-II information. Manufacturing the handheld tester was not hard; the challenge was creating a database that users could tap into. Back then, the appropriate Internet technology and usage "was a long way off," Andreasen says. Chen, now the firm's CEO, tried getting venture capital, but "neither the company nor the market was ready," recounts CarMD's marketing director Kristin Brocco. Instead Chen and Andreasen spent years compiling a list of the repairs associated with the trouble codes. The database is in fact the main strength of CarMD.

The firm now has several thousand customers, each of whom can register up to three vehicles. The company also maintains a phone system of live technicians for motorists who break down on the road and cannot get online. The device knows only what the car's computer tells it, so it may not help with pesky issues such as check engine lights that repeatedly turn on and off on their own. A trusted repair shop is still essential, but communicating with the mechanic no longer has to be a one-way street.