CITROEN C4 PICASSO Handbook page 130

Driving
Emergency switch-off
In the event of an emergency only,
the engine can be switched of without
conditions. To do this, press and hold
the "START/STOP" button for about
3 seconds.
In this case the steering column locks as soon
as the vehicle stops.
128
Electronic key not
recognised
If the electronic key is no longer in the
recognition zone when driving or when you
(later) request switching off the engine, a
message appears in the instrument panel.
F Press and hold the "START/
SToP" button for about
3 seconds if you want to force
switching off the engine (note
that restarting will not be
possible without the key).