service engine light

If in fact the cel came on because of a loose gas cap or dirt at the seal then tightening it was good but I believe you still need to clear the code and drive to see if it fixed it. I could be wrong about the clear the code part but another tech will clear that up. If the light comes back on then will likely be something else and yes should see what the code(s) are related to. Does not have to go to dealer for that. Can go to any shop that is qualified. By the way, alot of parts houses/stores will plug in and read codes and clear for free. Good luck. Auto Zone will for sure.