Here is what I have learned about this on my '07 Explorer:
The sensor is on the inside of the rim, not on the valve stem. So the magnet reset procedure will not work on this type.
The batteries go to inactive mode if the car sits for a long time, and reawaken after a couple of miles of driving.
If I completely remove the wheel and mount the spare, the fault still shows, even though the wheel is at home in my garage. In other words, if it's off the car, the system still shows low pressure. (This was the cause of my problem!)
When the batteries die (7 to 10 years, my mechanic says), they are about $100 each to replace, including removing and remounting the tire, and balancing the wheel.
Pennsylvania DOES NOT require the low-pressure system to work, but the mechanic faces a $20,000 fine if he disarms it. If it works when the car comes in for inspection, it must work when it leaves or he has to fix it!
To replace my blown tire, rather than spend over $200 for a new one, I purchased a complete brand new dirty but unused spare tire and wheel from my favorite junk-er auto salvage yard for $75, had my garage put the "new" tire on my original wheel and drove off. After 2 miles, the light went out.
Now I'm just waiting to see how long the batteries last. I'm at 85,000 miles.
