As I understand it,when this appears,it means you have an emission problem.
My gas cap was broken and I was running my truck w/o my gas cap on.
Would this cause the light to come on? Also,once it's on,will it go out on it's own? X

 

the CEL can mean a bunch of things- but yes, running your truck without the gas cap could have definitely triggered the CEL no, once it is on it has to be cleared by a technician, or if you have a tuner that reads codes like the edge or an xcal

 

you can do the scanner method or disconnect the Battery for ~ 8 min, hook back up, and follow the 2 drive cycle learning process.
http://www.obdii.com/drivecycleford.html

The battery disconnect will clear a DTC ( not a airbag code ) and clear the driving type that has been learned.

The 2 full drive cycles will re-run the tests, and set a DTC, if it was not the gas cap causing it.

A loose / damaged / missing gas cap can cause a DTC, as can a oil dip stick not seated correctly.

No technician or scanner required, unless the CEL icon is illuminated again after the 2 drive cycles, in which case, off to AutoZone or the likes that does free scans, to get the actual DTC number ( not what the guy thinks is wrong ).