What the heck is a EVAP system?

It stands for Evaporative emissions control. It keeps fuel vapors inside you fuel tank from escaping into the environment. 1996 and newer vehicles have a computer that monitors the components of the EVAP system and will set a code that will cause the check engine light to come on if it detects a leak or fault. Leaving your gas cap loose or off will certainly turn on your check engine light. When the cap is secured properly the light will go off again the next time your computer runs a system check on the EVAP.