Either your supply is providing insufficient voltage (i.e. cannot be below 12V) or insufficient wattage...
My power supply is 12,15V @5A, thats's more power than the original supply.
The voltage is not dropping below 12V while trying to switsch on. I used an oscilloscope to measure it.
Not sure what to suggest. There are small DC-DC boards advertised at some of the Car PC sites that you could try using to manage the power input better (example: DCDC-NUC, 6-48V automotiove power supply for NUC, 12V or 19V output)...
I have a DC-DC converter based on the XL6009, a very cheap module. I will check if it works today...
I investigated the problem a bit and now I think it's a problem of the switching power supply in the NUC. When I connect the power plug the voltage oscillating a bit at a frequency f.
If I disconnect the plug and reconnect it some times, sometimes this frequency is changing to 2*f. In this cases the NUC is able to boot and it works.
All in all, my NUC is not able to work at 12.15V. Perhaps it's damaged or the range in the datasheet of 12V-19V is not right.
The DC-DC converter based on the XL6009 is working fine for me. I calibrated a voltage of 16V.