Lexmark X736
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.
`296 Scanner Locked' Error on a Lexmark X73x Multifunction Printer (MFP)
Check this article to know how to troubleshoot a 840.02 or 843.99 scanner error code on a Lexmark X73x and X74x printer series.