Nikon D720 User Manual page 347

D
Image Sensor Cleaning
Using camera controls during startup interrupts image sensor
cleaning. Image sensor cleaning may not be performed at startup if
the flash is charging.
If dust can not be fully removed using the options in the Clean image
sensor menu, clean the image sensor manually (0 324) or consult a
Nikon-authorized service representative.
If image sensor cleaning is performed several times in succession,
image sensor cleaning may be temporarily disabled to protect the
camera's internal circuitry. Cleaning can be performed again after a
short wait.
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