I had a fuel economy problem on my truck (down to about 17 or 16 with a 2.4L) that improved by replacing the front O2 sensor. I was up to 22MPG on the last tank but probably average 20-21 overall now. This was at about 105K, and the old sensors checked fine, even in the ECU test where you count the number of lean/rich flips. I did a complete tune-up first with new OEM plugs, dist. cap and rotor, plug wires, fuel filter, etc... but still had the low MPG until I replaced the O2 sensor.
I bought mine from
www.sparkplugs.com - it was the Denso OE Equivalent and came with the correct harness and wire length. I assume yours is the V6? If so you'll probably have four sensors, two fronts and two rears. I'd try repacing the fronts first and see what happens. If you want to go with OEM you can get a good deal with
www.everythingnissan.com - best to call them at 1-888-216-5328 and ask for Joe. I wouldn't recommend going the cut and splice route with aftermarket sensors...
Good luck!
Heath