Before I went for my smog test, I plugged in my OBD-2 Code Reader / Scanner to my truck, found out that I had a vacuum leak. I looked up the OBD codes online and went down the list; checked all the hoses, clamps, etc. went through a process of elimination until I finally narrowed it down to the gas cap. I found that the rubber seal was in poor condition and not sealing properly and was allowing gas fumes and vacuum pressure to escape.

Solution: I went to the parts store and bought a new one and installed it myself.

If I had gone to the smog test station, my truck would have failed the smog test, I would have probably had to pay the mechanic for a secondary test to find the problem and they would have had to charge me for a new gas cap as well.