PO453 Code on 2005 Hyundai Elantra: Is It Gas Cap or a Broken Sensor?

Hi everyone! My 2005 Hyundai Elantra recently triggered the check engine light. After using an OBD2 scanner, I found a P0453 code — indicating an Evaporative Emissions System issue. I took it to my local Firestone mechanic, who said diagnostics alone cost $100. They didn't mention 'small' in their diagnosis, which made me suspicious. They claimed there was a specific part that was broken and would require nearly $500 to replace — but noted this repair depended on another component, potentially leading to additional costs of up to another $500. I decided to wait and monitor the situation. However, since my car failed Connecticut emissions testing, I have a limited time window to fix it or risk being unable to register the vehicle. A few weeks later — shortly after refueling — the check engine light turned off on its own. I didn’t clear it manually. When I scanned again, the P0453 code was still present. After clearing the code, a subsequent scan showed no errors. So here’s my confusion: Could this have been simply a faulty gas cap all along — even though the mechanic didn't mention 'small' in their diagnosis? If so, why did they say there was a broken part needing replacement? Also, could my scanner be inaccurate? Did it show P0453 and then disappear after clearing when the issue was still present? Would such an intermittent problem show up again quickly or would the scan return 'no codes'? I’d really appreciate any insight into whether this is a gas cap issue or something more serious like a faulty Fuel Tank Pressure (FTP) sensor. And how can I trust a diagnosis that shows a part failure but then no code appears after clearing? Thanks for your help — Diana