There are well over 1000 diagnostic fault codes that can be set. About half of them refer to things that could have an adverse affect on emissions, either out the tail pipe from the way the engine is running, or from vapors escaping from the vapor recovery system that all cars have had since the '70s. Those that could affect emissions are required to turn the Check Engine light on to alert you. One of those codes is "large evap. leak detected". That's the one a loose gas cap will set.
The place to start is by having the codes read. Many auto parts stores will do that for you for free. If you'd like a better idea of the things that are covered, check out this page:
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Saturday, February 9th, 2013 AT 9:19 AM