Canon EOS-1D X Mark III Advanced User's Manual page 66

Automatic Sensor Cleaning
Whenever the power switch is set to < / > or < >, the sensor is
cleaned automatically (which may make a faint sound). During the sensor cleaning, the
screen will display < >.
Even during sensor cleaning, you can stop cleaning and shoot immediately by pressing
the shutter button halfway ( ).
If you repeatedly turn the power switch to < / > or < > within a short
time period, the < > icon may not be displayed, but this does not indicate the
camera is malfunctioning.
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