The TPMS sensors must be carried over from the factory rims when new ones are installed. The following message may help someone who, like me, installed bigger tires on the factory rims. I have an '08 F350 with the TPMS and the factory front tires were rated at 65psi, while the rears were rated at 80psi. When I swapped on a set of 35 X 12.50 Toyos ( running 65psi) the idiot light on the dash came on and pretty much stayed lit from then on out. I checked the owners manual, and there are detailed instructions on how to reset your TPMS system. It goes something like...cycle the ignition on the vehicle twice, but do not start, then tap the brake pedal three times, etc. etc. Basically once the system is primed and ready ( the vehicle will beep the horn twice, or something like that), you'll go around to each tire in the correct order specified in the manual and hold the little grey FOB with the green button next to the bead where the tire sits on the rim on the side exactly opposite the valve stem. Press the button until the little L.E.D. flashes and move on to the next one. Before you do all of this, make sure that the tires are properly inflated to the pressure you want to run in them. The system will reset itself to monitor that new pressure. The warning light will then only come on if your tire pressure goes 5-10 psi above or below that mark. My instructions are just what I can remember off the top of my head, so definitely follow the manual to a tee. Hope this helps.
