I used to power my Pi 3B and 3B+ from and external shield board of my design, which features 5A power source, connecting +5V to pins 2 and 4, and GND to 6, 14, 30, 34 and 39. It worked just fine, however recently I wanted to upgrade to Pi 4B 4GB - and it no longer works, Pi simply won't start. When I connect it to my PC via USB-C cable, it starts just fine, but keeps showing under-voltage warning - no surprise there, since USB 3 only provides 0,9A as opposed to recommended 2,5A.

My power source is fine - like I said, it's 5 amps, so it's way more than required by PI and way more than USB port provides. I measured the output of my power source while Pi is powered on, and it's a perfect 5V.

There are two diodes next to USB-C port. When I power the PI from my PC, the green one shines continuously, however when powering it via my board, the red one shines and green one blinks a few times. I read the post speciffic to Pi 4 on the official forum:


but the green LED blink in a completely random manner.

Does Pi 4 no longer allow powering the board via GPIO pins? If that's not the case, then what might be the issue?

  • PoE hats supply power through the 5v pins, so, yes, Pi 4 does allow power from the 5v pins on the extension header (technically, they are not GPIO pins, they are power pins - some people are pedantic about that, I'm not one of those)
    – Bravo
    Aug 11, 2021 at 15:10
  • Like I said, Pi is not dead because it works just fine when powered from PC.
    – mag_zbc
    Aug 11, 2021 at 15:13
  • Fair enough - I misunderstood - anyway, there's no problem powering the pi 4 through pins on the extension header - I have 3 pi4's with PoE's doing just that - but still, count the blinks - it's usually a sign it can't boot
    – Bravo
    Aug 11, 2021 at 15:19
  • Yes, it can't boot, that's the core of the issue. The question is - why.
    – mag_zbc
    Aug 11, 2021 at 15:39
  • well, the blink pattern usually tells you, unfortunately blinks a few times is not one of the patterns that is documented - I wasn't getting you to count the blinks to be a dick
    – Bravo
    Aug 11, 2021 at 15:41

2 Answers 2


As a first guess, based on your description of "random" blinking of the ACT (green activity LED), I'd say that your RPi is booting, but your monitor is not cooperating when powered from the headers. This should be verified before proceeding with trouble-shooting.

And so, Step 1 is to test login to your RPi via SSH. If you've never logged into your RPi via SSH, you should familiarize yourself with how this is done:

If you do not recall adding an empty file named ssh to the /boot directory of your microSD card, do that first. For example: mount /boot, then touch /boot/ssh.

If you're able to connect/login to your RPi 4 using SSH, you'll know your problem is related to the HDMI hardware/firmware/software. We can proceed from that point, or you may wish to do a further search - perhaps similar to this one.


I have a few Pi4B, none powered via USB.

I generate the 5.1V from 12V used to power magnetic discs with a low cost Chinese voltage regulator which has a multi turn potentiometer.

My Pi's work best with 5.2V and can run the whole day without any glitches.

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