ok (im assuming there are NO exhaust leaks)since there is only 1 converter and it is a GM converter you put in.either one of the O2 sensors are bad or the new converter is bad.more than likely one of the O2 sensors is sending a wrong signal to the computer if that converter is new its pretty rare to have a GM cat bad right out of the box.the computer reads the O2 reading before the exhaust goes into the converter then it reads the exhaust coming out with another O2 sensor.if BOTH signals are the same it will set this code.stating the converter is now working or the efficancy is not working correctly .the front O2 should be reading rich/lean showing a lean rich mixture in the exhaust then when it goes into a good converter it should convert the gases to allot leaner so the O2 reading on the sensor that reads the out exhaust should be reading very slow and lean..there is a good chance the O2 sensors where damaged when it was run without the converter before you replaced the converter.like i said it is RARE to have a GM converter bad out of the box.but a few simple tests will tell you if its the converter or an O2 sensor causing this code.did i explain that to you ok?did you understand what i said?