CITROEN DS4 Owner's Handbook Manual page 75

Trip zero reset
 When the required trip is displayed, press
the thumb wheel located on the left of the
steering wheel or the button on the end of
the wiper stalk for more than two seconds.
Trips "1" and "2" are independent but their use
is identical.
For example, trip "1" can be used for daily
figures and trip "2" for monthly figures.
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Multifunction screens