The obd won't start if the arduino monitor serial is not open

if i upload the program and unplug the usb wire and then plug the obd adapter, it don't start communication …. but if then i plug the usb wire and open serial monitor it start to run fine… then i can unplug the arduino power wire and connect it again and have no problem… but if i disconnect the obdii adapter and don't connect the usb wire don't work… hope you can help me..

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