I think how it works is that the computer closes some solenoids in the tank under a certain condition, and then determines if the tank is holding the pressure, or leaking fumes. A loose gas cap would of course not allow the system to hold pressure. But the gas cap is not the only component and potential source of a leak. Perhaps there is another vapor leak in the system, or maybe the sensors that detect the pressure are going bad? My guess is that unless you have very expensive test equipement, you wont be able to figure it out in the driveway, and should consult a more competent dealership, especially if you are sure you have a good gas cap and tighten it properly.