I have a 1996 Ford Taurus with the 3.4L SHO engine. Recently the Check Engine light came on while I was driving, so I brought it into the dealership service department. They told me the reason the light came on was because the gas cap wasn't tightened all the way. I got the car back and the light hasn't come on since. The people at the dealer didn't give me much of an explanation as to why a loose gas cap would turn on the Check Engine light.
I've had many other cars and have even left the gas cap off and there were never any dash lights coming on.
Today's automotive computer systems are getting significantly more advanced than systems from just a few years back. Manufacturers are now using a somewhat universal type setup called OBD II (On Board Diagnostics). This includes more circuits, sensors, diagnostic capabilities and Trouble Codes, like the one that was stored in the PCM (Power Control Module) memory when you failed to tighten the gas cap all the way.
The technician who worked on your car came up with a code P0442 or P1442. These codes indicate a vapor leak in the fuel tank. The FTP (Fuel Tank Pressure Sensor) monitors tank pressure and transmits the data to the PCM to be analyzed in order to determine if a problem exists. If the pressures are abnormal due to a loose gas cap or a leak elsewhere in the system the PCM will turn on the Check Engine light and set the appropriate Trouble Code. This vapor leak will release harmful hydrocarbons to the outside air.
The reasoning behind this type of modern emission control advancement is to let the driver know there is a problem, so that he (or she) will have the condition repaired ASAP. Before the addition of the Fuel Tank Pressure Sensor, a car could be driven with a vapor leak in the fuel tank giving off damaging emissions into the atmosphere for long periods of time without anyone ever knowing about it.