Hi!
I'm new in this, I was using an ultrasonic sensor to move a car, I loaded the program to the arduino, I open "serial monitor" and the sensor data is displayed and the car moves.
The problem I have is that when I want to use an external source (a battery) the car does not move, apparently the sensor does not read any data. How could I solve this ?!
Thank you.
One can only guess, but maybe get a battery with power adequate for the purpose, i.e. not a 9v PP3?
Without seeing your program, your circuit diagram or the type of battery you are using it is a bit pointless offering suggestions.
Maybe the handbrake is on?
...R
Are you using a motor sheild? perhaps your jumper is on
infused:
Are you using a motor sheild? perhaps your jumper is on
It's chilly outside, this time of year 
...R