Don't worry about it. I've had like a zillion keys made for my X and Frontier and never once did anyone ask for a key code. If you need a spare key just swing by a dealership and have them cut you a few, or ask for some when you stop in for service. I also had one made at a hardware store and they didn't need a keycode either. So I'm not sure what the importance of having it is since nobody needs it to cut you a key.

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01 SE 4x4
Silver Ice