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Have you had a drop in temps? If so, remember ten degree drop equals one pound drop in pressure inside the tire. A 40 degree drop means a 4 pound drop in pressure, and if the tires are not inflated to proper PSI, the system will start signalling you.
Make sure the COLD tire pressure is set at 30 or 35 (depending on the tire), and see if that fixes the problem (I SAY COLD, don't drive on them and then check). If that does not solve the problem, you can try the reprogram, as Hummer Guy suggests, as explained in the owner's manual.