2003 Alero has catalytic converter problem but CConly 1 year old

The catalytic converter is indicated with engine light diagnosis but the catalytic converter was replaced last year, it was suggested O2 sensor may be problem, how would I know and is there more than one O2 sensor?

Keep in mind that many vehicles have two O2 sensors, one upstream of the converter and another downstream of it. I could see that a compromised old one downstream could report (via voltage generation, that's what they do and how they work) that the converter isn't doing it's job. If your vehicle has more than say 90,000 miles on it, they should be replaced unless you're not worried about excessive emissions, lowered fuel economy, or a converter being damaged gradually by improper fuel/air mixture due to a worn out O2 sensor.

Yes their are two sensors one before the cat and one after ,and you would need a computer diagnostic software to find out ,like scanmaster but for what they cost i would just fit two new universal sensors which are about 60 euros plus tax for a three wire one and 80euros plus tax for a four wire one ,inside the box is a colour coded wiring explantion as they are universal and the colour of the cables do not match but its easy to do a 22mm spanner and a wiring terminal block ,take about ten minutes to do .