I stopped this afternoon and filled the tank of my Sonata and I later noticed that my check engine light is on. I did notice that the filler cap does not click when I screw it back onto the filler neck. I do know that there is a label stating that I should tighten the cap until one click is heard and I have heard stories about loose fuel tank caps causing the check engine light to appear due to insufficient fuel tank pressure. Could I have a defective cap that is causing the problem? I will take it to my local shop tomorrow to confirm this or find out the problem for sure.