How does the 2004 Highlander determine that tire pressure is out of range?

The tire pressure warning system is designed to provide warning when tire inflation pressure is critically low while the vehicle is moving. The low tire pressure warning light comes on to inform you that such low tire pressure may hamper driving.This system is not designed to operate when the vehicle is stopped. therefore,check the tire inflation pressure regularly.The recommended cold tire inflation pressure, tire size and the vehicle capacity weight are on the tire and loading information label. Installed tires should be of the specified size. They should be of the same size and construction. If tires other than the specified size are used, or if tires of different sizes or constructions are mixed, the tire pressure warning system will not function properly. The tire pressure warning system does not function properly under certain circumstances. In the following cases, the low tire pressure warning light may not come on even if the tire inflation pressure is low, or it may come on when the tire inflation pressure is actually normal.  A compact spare tire, snow tires, or tire chains are used.  The tire inflation pressure is excessively higher than specified, or tire inflation, pressure suddenly drops due to bursting or other causes. The vehicle is driven on a slippery road surface such as rough or frozen roads. The vehicle speed is less than 30 km/h (19 mph) or more than 100 km/h (62mph), and the driving duration is less than 5 minutes. The tires differ in tread pattern or manufacturer. The tires are not the specified size.  The tread wear is very different among the installed tires.  Rapid acceleration/deceleration or sharp turns is continued.  Towing a trailer.