When you turn on the system, the node canisters complete self-tests.
A series of tests is completed to check the operation of components and some of the options that have been installed when the units are first turned on. This series of tests is called the power-on self-test (POST).
If a critical failure is detected during the POST, the software is not loaded and the fault LED is illuminated.
When the code is loaded, additional testing takes place, which ensures that all of the required hardware and code components are installed and functioning correctly.