Need to replace the stem and was told by the tire guy at costco it was a dealer part. Is this true and if so is there somewhere online to order a few?
http://www.fangars.com/images/valvestem.jpg

After searching the web all that I could find was aftermarket caps that had inflation indicators on them. Perhaps its my nomenclature.

Also are these easy to replace?

I had a split valve stem on a trip one time. Took it to a little mom-and-pop tire shop in a small town in rural Florida and the guy there had me fixed up in 10 minutes. He replaced the stem from stock and didn't dismount the tire to do it. Cost all of $7 for the repair. Gotta love those small-town shops. (Oh, and this was on a Saturday morning, too).

the reason they told you it was a dealer item is because its not a conventional vavle stem, its an air pressure sensor. you can get one off ebay for about $20 if your lucky, the dealer it maybe about $100.

Thanks for replies, was wondering how the X knew when the tire was low.
Hit a screw out in the middle of nowhere in New Mexico around Chaco Canyon and saw the light come on. Got to use the spare for the first time and even though all the tires were registering at the right psi the light stayed on.
Bought two new tires at costco and two more next week before their sale ends.
Would like to have some stems in the case they can be easily installed if needed. Its a long way to and from anywhere most places the wife and I like to go when we can.

i would not recommend installing them yourself unless you have access to a tire machine and actually know what you are doing. mounting the tire or breaking it down off the rim can fuck one of those type of valve stems up quick. after that part is done then you have to program the xterra to recognize the new sensor, i would say go to a dealer to get it done since they have the most "direct hands on experience" with the installation and programing.