Assuming you have the 4.3 engine, bank 2 sensor 1 would be the O2 sensor threaded into the exhaust pipe directly after the passenger side exhaust manifold.
Assuming this is a 4.3L engine, bank 2 sensor 1 would be the first O2 sensor located in the exhaust header pipe on the passenger side of the engine.
it is the rear sensor bank 1 (the one after the converter)
That would be the sensor after the catalytic converter.
Need more info, is this the 4 cylinder or 6 cylinder engine and which sensor on bank 1.
Assuming this is a V-6, bank 1 sensor 1 would be the first O2 sensor in the exhaust on the drivers side of the engine.
Bank 1 sensor 3 would be the O2 sensor after the catalytic converter.
That should be the sensor located after the catalytic converter.
That would be the O2 sensor after the catalytic converter.
Bank 1, on a 4.3 L engine would be on the drivers side.
That should be the O2 sensor just before the catalytic converter.
Neither, that should be the sensor after the catalytic converter. Bank 1 sensor 1 would be the first sensor on the drivers side of engine. Bank 2 sensor 1 would be the first sensor on the passengers side of engine. Bank 1 sensor 2 would be the sensor after the catalytic converter.
O2 sensor heater circuit malfunction (bank 1 sensor 1). You probably have a burned out Bank 1 Sensor 1 O2 sensor. Also check the wiring at the sensor for damage.