Only an advanced code reader will read all of the codes. The generic obd2 readers won't grab everything. My superchips will read everything from o2 sensors, disconnected air temp sensor, "radio missing", steering wheel off center, high fog light circuit current, etc. You can't pull those codes on the dash or with a generic reader.

On the topic of the gas cap- I've intentionally left mine off to see if I can get the 'gas cap' message but I didn't get that or a CEL so I put the cap back on (after 2 days of driving).

 

Warning went on this morning again. Did not come on after work and seams to be off. Thanks for everyone's advice. Sure is nice to have so much experience and knowledge at your finger tips.