What is CTD sensor on Fuji Xerox?
The Color Toner Density (CTD) Sensors measure the optical density of Toner deposited on the Marking Unit during the calibration cycles of the printing process. One sensor is located at each side of the Marking Unit to insure that the density of the Toner is consistent across the belt.
https://www.google.com/search?q=Fuji+Zerox+with+is+saying+CTD+sensor+error
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SOURCE:Fujitsu fi 5120c e2 error
See page 25 of:
http://193.128.183.41/support/scanner/manuals/fi-5120c_gs_p3pc-1312-01en.pdf
"E2" is "Trouble with the optical system (front)."
That doesn't solve your problem, but at least it narrow downs the focus of your problem.