The related fault codes refer to a "small evaporative emissions leak" or a large one. Codes never say to replace parts. The loose gas cap is only one of many possibilities for a large leak. You don't replace the cap for that; you tighten it. If the cap is tight, you have to check the rest of the system. Start with a visual inspection of all the hoses, valves, and clamps. If you don't see anything obvious, a smoke machine is the best way to find it. You inject a white, non-toxic smoke at 2 psi and watch where it sneaks out.