Dear Car Talk:

I have a 2006 Pontiac G6. I have had a continual problem with the computer telling me to check my gas cap. After that message comes up, it will then display the motor symbol. It continues to run fine afterward, but it will not pass inspection. I've replaced the gas cap; I've had it in the shop, too. But so far this problem continues to plague the car. Please help! -- Bill



What it's telling you, Bill, is that the fuel tank is not able to hold pressure. The fuel tank is supposed to be pressurized so that gasoline vapors can be controlled and captured, rather than released into the atmosphere.

If you're not convinced that we need to control fuel vapors and smog, Bill, take your next vacation in Beijing.

Fortunately for us, since the 1970s, all gasoline cars have come with something called an "evaporative emissions system." That system allows gasoline vapors that develop in the fuel tank to be stored in a charcoal canister, until they can be combusted next time you start the engine.

So my guess is that you have a leak somewhere in your evaporative emissions system, and that's why the tank is not holding pressure.

The gas cap is a common source of a pressure leak. As you can imagine, sometimes people forget to put the cap back on. Or they don't tighten it enough, or they screw it on incorrectly. But if you've tried a new factory gas cap and still get that message, I think it's fair to say that the gas cap itself is not the problem; the pressure is escaping from somewhere else.

So you need to take the car somewhere where a good mechanic can put the scan tool on it and figure out where the pressure leak is. It could be a solenoid that's malfunctioning. It could be the charcoal canister. It could be a rust-perforated filler neck. Or it could be something as simple as a hose that's old and cracked.

But something is keeping your gas tank from holding pressure. That's what's turning on your Check Engine light (the motor symbol on your dashboard) and preventing you from passing inspection, despite the $50 you keep trying to slip the inspector, Bill.