Doesn't turn on and says to download custom OS.

You want to press VOLUME UP, power and home buttons. Hold until you see Samsung then release. You are booting up in download mode by mistake.

"Our phones aren't exactly similar so. What works for the other might not work for you. I have a Samsung Galaxy Tab 2 10.1. Note that what I have is a TABLET, not a phone. According to the Samsung's site Help category,

in order to go to the selection of Resetting (i.e., Wipe cache partition, factory reset, reboot system) I must hold the POWER BUTTON & VOL. UP together.

However, holding them 'caused my device to head onto ""Custom Os installation"".

I figured out, what if I try holding the vol. down instead of vol. up?

Now, that was when it went to the selection of Resetting.

So, in simpler words: Try changing it up.

What worked for me:

Hold POWER BUTTON + VOL. UP = CUSTOM OS INSTALLATION

Hold POWER BUTTON + VOL. DOWN = ANDROID FACTORY RESET SELECTION

Remind you, this is what worked for my Samsung Tab 2 10.1."

Try holding the power button until it powers down (more than ten seconds). You've booted into download mode. Once powered down, plug it into the charger and leave it alone for a few hours. Then attempt boot. Let me know what happens.

Hold the power button + volume button down + home button all at the same time

if you held home volume up and power and it said download custom os then shut it off again and hold volume down home and power

"remove the battery this will reset your phone

i hope this help you out."