Currently, energy saving policies are fueling a broad research effort to reduce power consumption and increase energy usage efficiency. Private cars are a key energy consumer and constitute a significant source of greenhouse effect gas emissions. The advent of information technologies and the proliferation of wireless low-power communication technologies have allowed the appearance of several energy saving solutions. One example is parking assistance systems for large parking lots, which allows user guidance to the free parking places. This paper describes one of these systems implemented with an IEEE 802.15.4 network.