Intermittent 900 firmware error appears on the printer and scanner is too hot at some point.
Turn off the printer and power it back on after one minute.
Check the printer if the error message still appears.
Check the surface of the scanner and verify if becomes too hot.
Here are steps to troubleshoot a an or an error on a Lexmark X264, X36x, or X46x series printer.
Follow the steps in this article on how to troubleshoot a communication failure issue on a Lexmark printer.