One of the most fundamental issues with the Pi may be that it fails to boot up properly at all. Usually, the Raspberry Pi logo will appear, then the screen will transform into a block of rainbow color as the Pi self-tests its graphics processor. Then occasionally the Pi will either display scrolling text on a black screen or simply reboot repeatedly and fail to load the OS.
The most common reason for this is that your power supply is simply not providing enough juice. The Pi is powered by a 5V micro-USB cable.
The amount of current required will vary depending on your model. For the latest Raspberry Pi 3 Model B, use a 2,500mA supply to make sure the Pi switches on and that all four USB ports are powered. For a detailed list of the Pi’s power requirements visit https://www.raspberrypi.org/help/
This Micro USB 5V Supply is available for just 8.99 USD ( Buy Now )
Speaking of USB ports, bear in mind that the power requirements for the Pi will vary
depending on what devices are attached. The Pi will usually display a lightning bolt symbol at the top right of the desktop if it is underpowered but can also simply reboot if too much current is being drawn.
Ideally, you should buy an official Raspberry Pi power supply from an authorised
dealer such as the Pi Hut. Where possible, try to connect the Pi directly to the adapter itself rather than through a USB hub.
Although insufficient power is the most common reason that a Pi is stuck in a boot cycle, there are other reasons why this may occur. If your SD card has been formatted incorrectly or an update to Raspbian has gone awry, the Pi may also reboot.
Try installing NOOBS to your microSD card and reinstall Raspbian if changing the
power supply doesn’t help.
Fix Stuck on the Rainbow Screen
We mentioned opposite that on booting up, the Pi runs its own GPU (Graphics
Processing Unit) self-test, which appears as a dazzling rainbow colored screen. Occasionally the Pi may hang on this screen and refuse to load the OS.
This can happen when the Raspbian kernel image is corrupted. An easy way to test
this is to use your computer to install Raspbian onto a new microSD card, insert this new
card into the Pi and try to boot.
See www.raspberrypi.org/documentation/installation/installing-images for more
information on this.
While this solution works well in theory, it can be a nuisance to have to configure your Pi all over again. You might also have personal files on the original microSD card. If so, first back up your data: power off, remove the Pi’s SD card, insert it into
a card reader then connects this to a computer.
The card’s second partition should contain your /home/ folder. Make a copy of this to a safe place such as your computer’s desktop.
To try to repair Raspbian’s kernel, you must run a system update. If the Pi will not boot
normally, it’s best to try to connect via SSH.
For recent versions of the Raspbian OS, to enable access this way you need
to save a file named ssh onto the SD card’s /boot partition.
If you’re able to connect to your Pi via SSH, run the following two commands to attempt to reinstall the Pi kernel:
sudo apt-get update
sudo apt-get install raspberrypi-kernel
Once the update has completed, also edit the config file with the following:
sudo nano /boot/config.txt
Add the text boot_delay=1 on a new line. Press Ctrl + X, then Y, then Return to save and exit.
Now try to boot the Pi again. If the OS still won’t load, you’ll need to run a fresh install of
Raspbian on another SD card as outlined above.
Hope this article “How To Fix Raspberry Pi Boot Problems” helps you to Fix Raspberry Pi Boot Problems and Raspberry PI Stuck on the Rainbow Screen issue.