Mine (old Doran) goes off when the pressure drops by a preset percentage - 10 or 15, can't remember exactly. The display on mine is blank until you press it for a reading or until it alarms. It tells you which tire...but only if you are careful about putting the same sensor on the same tire each time. Since I only have sensors for the 2 trailer tires, I am not careful about this. I take them off between trips because the manual says the sensor batteries are conserved if they do not read pressure.
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