OBD mode function: vehicle speed, water temperature, voltage, fuel consumption, speed, single mileage, total mileage, ulative driving time, driving direction, number of star searches, alude, system time, intake pressure, oil temperature, air-fuel ratio, turbine pressure, intake pressure, acceleration test, brake test, clear fault codes, read data flow. GPS mode function: vehicle speed driving time satellite time alude satellite number driving direction single mileage voltage acceleration test braking test. HUD short for Head Up Display, The main function is to display the car dashboard information (e.g. speed, RPM, water temperature, voltage, trip distance etc. ),and keep the driver eyes always on road to avoid the dangerous of lowering down head to read the dashboard. Best service and best product quality