There's a new one going around and I figured I'd warn you.
As you know, all cars available for purchase in the United States are controlled by computer. Using a bunch of sensors, the computers adjust the air/fuel mixture constantly to assure optimum performance and fuel economy. One of the systems monitored by the processor (in cars built after '96, earlier for those tree-huggers in California) is the low pressure side of the fuel system. Namely the fuel tank and the atmosphere in it above the surface of the fuel. The air in your fuel tank is concentrated with unburned hydrocarbons, basically evaporated gasoline.
These hydrocarbons are pollutants, so the pressure of the air pocket in your fuel tank is measured and the vapors are recycled into the fuel system, to be burned in the engine instead of vented into the environment. In other words, the processor knows if your gas cap is loose or missing, or you got a hole in the tank.
When the computer sees this condition, it records a trouble code and turns on the 'check engine' or 'service engine soon' light on the instrument cluster. You see the light and you come to me and say 'fix it'.
It's first question I ask when regular customers pull in with the light burning and the car seems to run fine. "When was the last time you got gas?" Seems that some of the paper assholes who run ripoff shops have figured out a new scam. It generally happens at full-serve stations.
You pull up to the pump and tell the nitwit what you want. "20 bucks regular, please." And dipishit fills you up, reinstalling the cap when he's done. But he doesn't tighten it all the way. Rat bastid leaves it a little loose and you drive away. You go a couple miles and ta-da, the light comes on. Where do you go? Back to the dipshit who sold you gas. He says, 'we can scan the computer and see what's wrong', and charge you $75 bucks for the scan. 'You can pick the car up later', he says.
You come back at 5 o'clock and you get the bill for the scan and for a coolant sensor, or some other not-too-expensive one so you don't feel like you're getting ripped off too bad. All he did was tighten your gas cap as soon as you left and cleared the evap code from your computer. Boom, no light, you're happy, money well spent.
You think you're getting a deal because what he's told you is PFM (Pure Fucking Magic) to you. He's got you thinking thousands and $125 doesn't seem so bad. So, heed my words, Lugnut.
If your service engine light comes on within a couple hours of you getting gas, tighten the cap util you hear a few clicks. Something to listen for if you go full-serve when the spooge puts your gas cap back on. If it don't click, it ain't tight. Boy, that sounded Johnny Cochran-esque. The light should go out on its own in a few hours or over night. If not, have a guy you trust look at it.
About a half dozen of our older customers have fallen for this from the same guy near us. Harry's debating paying him a visit and explaining the rules.