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.
Lexmark CX82x/86x, MX72/82: 843.00 Scanner Error Message
Lexmark CX/CS8xx and XC/Cxxxx: 843.01 Error Message