Dash indicate light question

Thanks for reading. I am trying to figure something out.

I know the engine light will come on for just about any sensor related issuse.
My question is on the check engine light. For clairifaction the engine light with the word check in it.

The code is spn 2659 fmi 0. I'm not asking about the code just the light. While warming up the truck or cooling off after the days drive. And sometimes during the ride the check engine light will shut off. But it comes back on randomly.
It will show the same code with the check engine light on or off.
Again i am confused. If its the same code, why does that light come on and off but the code doesn't go away.

I know what the code is for and will be getting to it this week. I just don't understand the on and off part.

 

Thanks every body they all answered it thank you