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The little red light in my 1993 Plymouth Grand Voyager said “Check Engine.” I appreciated the suggestion. But I had no idea what it meant.

The van, you see, had been a gift from my wife’s sister. It had 187,00 kilometers (116,000 miles) on it when I picked it up in Chicago for the long drive to Boston. Halfway back the engine almost caught on fire; when I arrived two days later, I discovered that it needed nearly $5,000 in repairs. Three days after that work was done, the transmission died.

All in all, this “free” van had cost me nearly $8,000, and now something else was wrong. What was it? I didn’t trust the dealership to tell me what was really going on: I felt that they were dishing out the bad news one broken part at a time, so that I wouldn’t realize the full scope of my problems. So instead of turning to the experts, I turned to the Internet. I was troubled by what I learned-not about the state of the old van, but about how limited my access was to information about a product that I owned.

A lot of people are intimidated by the idea of trying to repair a modern car. What’s so discouraging, apparently, is the “car computer”-the electronic brain that controls everything from the emissions system to the battery charger. People who once had no problem doing tune-ups with strobe timing lights see these electronic boxes and just give up.

But in fact, the car computer is your friend-constantly monitoring whether anything is going wrong.

At the suggestion of a friend, I typed the keywords “Voyager” and “Diagnostic Trouble Codes” into Google. This landed me at a Web site that offered step-by-step instructions on how to get my car’s computer to spill its secrets.

Different cars have different “secret handshakes” that you need to know to extract the diagnostic codes. On some GM cars, for instance, you turn on the car’s ignition then hold down the “Off” and “Warmer” buttons on the climate control system until a special light appears on the instrument panel. For other cars you need a device called an onboard diagnostic (OBD) scanner; these gizmos cost anywhere from $20 to $250. For my Plymouth van, you read the codes by turning the ignition switch on and off two times in a row, turning it back on, and counting the number of times that the “Check Engine” light flashes.

In theory, knowing these codes should let you fix your car just the way the pros do. Just ask the car computer what component is malfunctioning, and then replace it. And if the computer can’t figure it out, you replace the computer itself! This simple logic will let you diagnose the vast majority of common car problems.

But there’s a problem with this approach: you need to know those diagnostic codes. Without them, you’re nowhere. And as it turns out, even though you’ve paid tens of thousands of dollars to buy your car, you have no right to know what those codes really mean-or how to get them out of your machine.

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