Why does the low tire pressure light come on when all tires are full on a 2005 Nissan Frontier and how is it fixed?

I would check your owner's manual for the Correct Tire inflation used for your vehicle. Many people make the mistake of using the tire pressure on the side of the tire as the correct inflation pressure, which IS NOT the correct one. That is just the max. inflation pressure for the load carrying limit for that specific tire as marked on the tire itself. If your tire pressure is reading correct then, I would take it to the dealer, could be a faulty sensor. Good Luck.