Honda: Check Engine Light, p1456, honda

Question
Hi Gary,

I have a 2000 Honda Civic. On Sunday November 15, I was filling up my car with gas, and after I filled the car up and then turned the car on the yellow check light came on. The light had never come on before and was not on when I was pulling into the gas station. After that I drove back to my college which is about 2 hours away from my home and did some research and found that this could be because of a loose gas cap? So I went out on Tuesday the 17th and opened the gas cap and I heard a release of pressure or gas and then tightly put the gas cap and made sure I heard about 3 or 4 clicks. Was the release of pressure or gas a sign that I had not correctly closed the cap in the first place? I honestly don't remember if I did it tight enough when I was initially filling up my gas tank. Now I didn't touch the car till yesterday and Friday to see if the light was still there and light is still there, so I was wondering how long does the light stay on if in fact if it was a loose gas cap? Also on Sunday when I was driving back I noticed that whenever I would come to a stop the car would shake/vibrate/rattle whichever term you prefer. I must have stopped about 4-5 times so each time that shaking happened. I noticed that vibration happened much less on Friday only about 2-3 times total, and not at all today on Saturday Could this be a sign that it was a loose gas cap since the car is not shaking now? However, the light is still on though. Please help.

Answer
If a loose gas cap was the culprit, the car would have a P1456 evap system leak code stored in the PCM. A P1456 code would not turn on the check engine light until the PCM detected a problem during two different drive cycles. I could get pretty technical on this code, but I need to know if that is indeed the code.