ACT LED stops blinking, Raspberry Pi booting and reading from card as always

Question

I couldn't find a similar situation. I noticed that the ACT LED on the motherboard of my Raspberry Pi 4 stopped working. I used to see it blinking while restarting the Pi, now it's gone. It is possible that the LED is broken? Is there way to test it? I tried to measure it by the voltmeter (multimeter) but it shows something about 0.45 V.

Is there any selftest or something?

I've checked 2 SD cards, same.

Edit: I ran an external led on GPIO and it's flashing.

In config file /boot/config.txt I've added this line:

dtparam=act_led_gpio=25

Answer 1

I ran external led on GPIO and it flashing.

Presumably you mean, you found it flashing at points when the ACT led wouldn't.

The problem with this is that the ACT led only flashes when there is significant activity on the SD card reader. This is why it flashes erratically when booting up.

If something goes wrong at boot, it may flash in a regular pattern to help indicate the problem. Otherwise, other than flashing when there is SD card activity, it just flashes a sort of heartbeat once/second, but this flash is extremely brief and only noticeable in dim light or if you are looking directly down at it.

Hence, if what you notice with the led on GPIO 25 is a 1 Hz pulse, that's probably because you did not notice this with the (presumably much smaller) board led.

I couldn't find similar situation.

My point is that's because what you describe is totally normal.

Answer 2

I found the reason, the resistor connected to the LED fell off (I checked the diagram is 1kOhm). I had to accidentally hook it or it was poorly soldered to the board.