what do i need to replace , i believe its a sensor

Car has massive loss of power, seems as if the motor is not firing on all pistons. If it is, their not in order, running very rich(gas smell). When it originally started I preformed a tune - up on car . It then ran normal for 1-2 days and then everything I put above. Also if I get the car to high speeds, it does not seem as anything is wrong.