The fuel door will lock when the car is locked. If you lock the car while the fuel door is open, you will not be able to close the fuel door until the car is unlocked.
To open the fuel door on Tucson, make sure the driver’s door is unlocked and then press the fuel door at the three o’clock position to spring it open.
When you are finished refueling, make sure the fuel tank cap is securely tightened by rotating the cap clockwise until it clicks at least one time. Then simply close the fuel door and press on the centre-right to latch it again.