Volvo S80 Oxygen Sensor Issue
 

Model: 2000 Volvo S80 2.9L non-turbo 108Kmiles

In 2008 (around 5000 miles back) I installed two new Bosch 17036 Oxygen Sensors (O2) upstream or before the Catalytic Converter. I reset the codes and passes emissions inspection no problem. Less than two years later, the light is back on, throwing a code of P0161 ( P0161 O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2)).

Is this the aft O2 sensor? If so how do you check? Would another issue be causing this or is most likely the sensor is bad?