I am getting a intermitant Check Engine Light why?

Had car code checked at local auto store for check engine light. Replaced the rear oxygen sensor on catalytic converter. Check engine light appeared again several days later had code checked again diagnosis was to replace catalytic converter. Have not done this yet due to the check engine light will go away when my fuel tank is below half of a tank of fuel. When the tank is full the light stays on until about a half of tank of fuel remains. Car's performance is not affected runs great no overheating. Could this be caused by a bad fuel cap, fuel injectors dirty, or fuel filter? I don't want to keep replacing parts that do not eliminate my problem.