Just a side note - my pressure sensor clicked on about a week ago. At the time I didn't have a pressure gauge in my Xterra (and for some reason, no place in NYC seems to seel them), so I stopped by a gas station and added a bit more air to each tire. The sensor didn't go off. When I final was able to get to an Autozone a week later to get a gauge - I found the front passenger tire at 80psi, and others at 60psi. (explains the bouncy, unstable ride I've been having lately).

Bottom line is that the sensor also seems to go off when you have too much pressure in the tires or there is a very unequal distribution of pressure between tires. It was odd, because I hadn't added any air since rolling off the dealer lot, but I guess temperature changes could have come into play.