A rental/car-share (RCS) vehicle access and management system and method, in some embodiments, utilizes barcodes, QR codes (or NFC/RFID), GPS, and a mobile app coupled with a wireless network to enable customers to bypass the reservation desk and pickup and drop off reserved RCS vehicles using a mobile phone, tablet or laptop. The QR code, RFID, or NFC communication with the mobile application allows for identification of the vehicle by a mobile application, which, if authorized, can access the vehicle via a temporary access code issued by remote servers. The remote servers and/or mobile application communicates with a control module that plugs into the on-board diagnostics module of the rental/car-share (RCS) vehicle and includes a host processing unit with a processor, an accelerometer, data storage, a GPS with internal GPS antenna; a wireless modem with internal antenna, and CAN bus transceivers connected with the processor, and a USB programmable interface.