How many miles on it?

 

104,000 give or take a few hundred

 

If it hasn't been replaced, it is probably time for a new one then.

 

Replace it.

 

Id swap the new O2 sensor you bought to the front and the other to the back if the cords will reach. There is no difference in the front or back sensor save possibly the cord length, and the rear only trips a CEL if the cat is malfunctioning. I also remember reading somewhere that when the PCM sees a colder temp reading on the coolant temp sensor, it pulls a pinch of fuel at idle to make the engine heat up faster until the truck reaches closex loop operation. At any amount of throttle tip in over idle, the PCM uses standard values to calc fuel trims.