CITROËN C4 Handbook page 34

Instruments
Service indicator zero reset
After each service, the service indicator must
be reset to zero.
If you have carried out the service on your
vehicle yourself:
F switch off the ignition,
F press and hold the ".../000" trip distance
recorder zero reset button,
F switch on the ignition; the distance recorder
display begins a countdown,
F when the screen indicates "=/0" and the
spanner disappears, release the button.
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Following this operation, if you wish to
disconnect the battery, lock the vehicle
and wait at least five minutes for the
zero reset to be registered.