2003 Ford Windstar 02 sensors?

Hi,
I took my van into Advanced Auto for a diagnostic test for the check engine light, they told me I needed a "rear 02 sensor". I bought and replace the sensor that I thought was the right 1 same problem! Also I could only fine 1 sensor not 2. It was on the catylic convertor is there another 1 and where is it? Also is there something else that will make the check engine light come on?