Why can the absence of a simple fuel tank cap (http://www.drivewire.com/part/fuel-delivery/gas-cap/) cause the check engine light to illuminate? The other day I filled my Pontiac Vibe with gas and now the check engine light is on and the gas cap is gone. I must have set it down at the gas station and left it there. Did I damage my car?

Chill. Your car is not damaged. The fuel system in your car is sealed that traps harmful gas fumes. If the gas tank cap is left off then gas is able to escape from the gas tank. When that happens, a self-test of the performance system detects this leak in the EVAP and the check engine light comes on, and the code is stored. It is just a false EVAP System failure code. Just buy a new gas cap.

good job guys
thanks