Dear Tom and Ray:

I have a 1996 Oldsmobile 88 with 66,000 miles on it. At about 50,000 miles, the "check engine" light came on and stayed on until I took it to the dealer. $58 later, they told me the problem was a loose gas cap. They turned out the light, but it came on again about 5,000 miles later. Not wanting to be their cash cow, I ignored it, and it went out on its own a week or two later. Last week, it came on again for a few days and then went off. I hate to make these $58 trips, but am I damaging anything by ignoring this "on and off" problem? -- Morry

RAY: Well, you might be damaging the environment, Morry, if not your car.

TOM: The "check engine" light comes on when one of the car's electronic sensors detects a problem. And most of the sensors are related to the car's pollution-control system.

RAY: There are a few "check engine" problems that could cause expensive damage if you don't fix them, but many are non-emergency items and can be taken care of at your earliest convenience.

TOM: For $58, your mechanic "scanned" your car (read the stored trouble codes in the computer) and found that the pressure sensor in the gas tank was indicating low pressure. The reason this is a problem is because it indicates that gasoline vapors are escaping. And a loose gas cap could cause that.

RAY: Needless to say, seeping gasoline vapors are bad for the environment -- and bad for anybody who happens to be lighting up a Tiparillo near the back end of your car.

TOM: So here's what I'd do next, Morry. I'd take another shot at the gas cap. Maybe it's loose because it has a bad seal. So get a new one. It costs $10 if you buy one. Less if you steal one. And it's worth a try.

RAY: If the light continues to come on, then you need to scrape up another 58 bucks and have your car scanned again. The "check engine" light could be coming on for a completely different reason this time. And you won't know that unless you plug it into the computer and scan it.

TOM: If it's still pointing to a pressure problem in the gas tank, then it's probably a leak in your evaporative emissions system. And your mechanic will have to address that. Good luck, Morry.