Cadillac Repair: 1996 DeVille DTC code PCO139, emissions systems, oxygen sensor

Question
Recently have a check engine light for Emissions systems.  Code check reveals a Current PCO139 code.  I believe this is an Oxygen Sensor fault.  Is there anything I should know about before attempting to change?  Since there are multiple sensors, how do I find the one specified for code 139?

Answer
Replace o2 sensor #2 on Bank 1 of the exhaust system.

The sensor is the one just in FRONT of the catalytic converter.

Once you replace it. You MUST CLEAR (erase) the codes via the climate control panel ONLY. Once BOTH codes are cleared. start the engine and see if the check engine light illuminates.