Hyundai Ioniq (AE): Evaporative Emission Control System / Fuel Filler Cap. Description and operation
A ratchet tightening device on the threaded fuel filler cap reduces the chances of incorrect installation, which seals the fuel filler. After the gasket on the fuel filler cap and the filler neck flange contact each other, the ratchet produces a loud clicking noise indicating the seal has been set.

Replacement1.Turn ignition switch OFF and disconnect the negative (-) battery cable.2.Remove the filler-neck assembly(Refer to Engine Control/Fuel System - "Filler-Neck Assembly")3.