Inaccurate tire pressure monitor

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My Tire pressure monitor is very inaccurate, and the car display and Onstar tells me that I am driving on three deflated tires. Yet, the tires are properly inflated when checked by the dealer with a pressure gage. The car is only 6 months old, and this has been a problem since the car was new. The dealer doesn't know how to fix it. There have been many other computer problems with this car, but after many visits to three different dealers, most of the other problems have been corrected.