p0036 code on 2004 Buick Century

replaced sensor 3 times light comes back on why

2 answers
How did you know it is P0036? How did you verify it is the sensor and which sensor are you replacing? http://www.jaysauto.net if you need more help.
bank 1 sensor 2 o2 sensor o2 sensor behind converter it only has two sensors right you put new sensors in car