2001 Windstar: Low Tire Pressure Warning Light

I'm getting an intermitten warning light that indicates that one or more of my tires has dropped in pressure. This is happening both before and after
I replace all four tires. The only common fact is the light comes on after about 80 miles of interstate driving. The pressure in all four tires are equal and at the recommended pressure (~44psi). Any suggestions on where to start looking for the problem; bad sensor (where are they located)?
Thanks. Charles Holland.
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