I took my car to Autozone because my check engine light came on and my car started to tremble when I turn the car on and continued while I drove it. When they checked it, it read 7 codes...6 meaning spark plugs need to be replaced. When I bought the plugs, they told me that they are already gapped. I think I can do it....doesn't look hard at all. Any advice before I "go in"'?