i have a 2001 hyundai elantra when i put gas in it it wont start unless i sit there and let the gas settle when my tank gets to half the gas light flashes on constantly someone said i need a oxygen sensor and someone said it was the catalytic converter which one is it

Optional Information: 2001 Hyundai elantra
Already Tried:
tried to get it fixed but they said wait until engine light comes back on