Abstract: A tire pressure warning system, including a valve stem-mounted tire pressure detector, including means for sensing the tire pressure and means for transmitting an encoded signal when the tire pressure drops below a predetermined level, and a receiver that can receive and recognize the encoded signal, differentiate between various encoded signals, and provide a warning signal to the vehicle operator. The inside of the tire pressure detector is hermetically sealed; inside the sealed portion is a unique on/off switch that can be externally actuated without compromising the seal.