Porgramm does't start if 12v power only is used


my programm does only start when I open the serial monitor or reset the arduino with the reset button, but I would like the arduino start working as soon it gets power. Does anybody have a clue what I have could have done wrong? I alredy eliminated all Serial comm lines in my code.

The Arduino will boot up as soon as it gets power. Is there any other hardware involved that might need time to initialise? Maybe you have a timing issue at startup. What have you got attached to the Arduino, and what sketch are you running? What happens when you just power up the Arduino?

I uploaded "blink" so it is not the sketch. And I observed the same. I disconnected the arduino from my hardware and "blink" startet as soon I connected the power. So it must be the hardware I conected.

My hadware consists of some bottons with 10K pull up resistors, MOSFETS on the PWM pins with RGB LEDS and some potiometers on the Analog in pins. I also use the V in pin to power the MOSFETS PIN 13, 0, 1 are not connected. When I connect the board to the 12V input the "on" LED os on and the L LED is also continously on but no so bright as it used to be. Any clue?

Make sure the input is the right way round, outer ring -ve. Make sure your power supply has a DC output not an AC one. Make sure your power supply can supply sufficient current. Measure the voltage on the Vin pin, this should be 0.7 V short of the power supply voltage.