Dear Tom and Ray:

You might be familiar with how our military has paid outrageous sums of money for items such as hammers and toilet seats. Did you know that an unsuspecting car owner can pay 125 bucks for a new gas cap to make the "check-engine" light go out? It starts with the minimum 99 dollar diagnostic charge, followed by the 24.99 for the golden gas cap. As a thinly veiled gesture of benevolence, the service manager graciously put the 99 dollar diagnostic fee toward the "labor" charge of some other work I had him do, so I only ended up paying for the gas cap. Is it true that an improperly fitting gas cap can cause the check-engine light to come on? The service manager said that there is an "escaped fume detection system" that causes this alarm. Did he really sell me a 125 dollar gas cap? -- Saul

RAY: Yes, it's true, Saul. It's all true. A loose gas cap will trigger the check-engine light. In fact, it's one of the leading causes of illuminated check-engine lights.

TOM: If your gas cap isn't tight (if you haven't turned it until it clicks), gasoline vapors can escape. And since we -- as a nation -- have spent bazillions of dollars during the past 30 years trying to clean up our air, nobody wants YOU driving around releasing unburned hydrocarbons into the air and creating your own personal smog cloud. So the light is a warning that the fuel system is not properly pressurized, and that something needs to be fixed.

RAY: Now, is the dealer within his rights to charge you 99 dollar to diagnose the problem? I'd say yes to that question, too. When you come in with the check-engine light on, that means your car's computer has stored a fault code. But in order to read the code, the mechanic has to hook up the scanner and input a bunch of information about your car. That's going to take him a good half-hour just to find out what the computer says.

TOM: I suppose he could have guessed and checked the gas cap. And if he had found it loose, he could have sent you away and said, "That's probably what it was." But there are two potential problems with that approach. First, he could have been wrong. The check-engine light can be on for hundreds of different reasons. And in that case, you'd be writing to us complaining that your dealer didn't fix your car.

RAY: The second potential problem is that your check-engine light could have stayed on. In lots of cars, once a problem is corrected, the light will go off by itself after somewhere between 20 or 50 starts. But on other cars, like VWs and Audis, the light never goes off until your mechanic scans the computer and resets the system.

TOM: So, Saul, I actually think it was a nice gesture of the dealer to waive the diagnostic fee. He didn't have to do that. So bake him some brownies, then go back and tell him you're sorry for overturning his tool box and trashing the waiting room.