2000 sierra obd code p0137

i have a 2000 sierra and the engine light is on and the code is p0137: 02 sensor circuit low voltage ( bank 1 sensor 2). can you tell me where this is or how to replace it. thanks
That is the O/2 sensor after the converter. It's likely a bad sensor but could also be a leak in the exhaust before the sensor introducing air into the converter.
 

will check it out.
To add: If in a rust belt the wires can fail at sensor! Can't really fix that if the pulled out - replacement is best and OE grade seems to last longer on the dang things!