Why check engine light is still on?

What is your question?

I have a 2005 Kia Sorento and the check engine light is on. I had it checked and it was reading that the O2 Oxygen Sensor need to be fixed. I had the O2 Oxygen Sensor fixed but the check engine still on and saying that the O2 Oxygen Sensor is need to be replaced. There must be another issue that they are missing. What could it be?

What makes this problem better or worse?

Not giving good gas mileage like it suppose to

How long has your 2005 Kia Sorento had this problem?

6 months