The 556U Interface module is used when installing remote start products in vehicles equipped with an RF (Radio Frequency) based immobilizer system. This type of system uses a small chip imbedded in the ignition key, called a transponder, to transmit a very low powered RF signal. The signal is picked up through an antenna (coil) or around the ignition switch which is then sent to the immobilizer's transceiver. Once the correct signal is received, the immobilizer will enable the ignition and/or fuel systems.