CITROËN C4 Picasso Owner's Handbook Manual page 226

Safety
Before reinitialising the system, ensure
that the pressures of the four tyres are
correct for the use of the vehicle and in
line with the recommendations on the
tyre pressure label.
The under-inflation detection system
does not give a warning if a pressure is
incorrect at the time of reinitialisation.
Snow chains
The system does not have to be
reinitialised after fitting or removing
snow chains.
226
Reinitialisation of the system is done with the
ignition on and the vehicle stationary.
From the secondary page of the " Driving
assistance" menu:
Press " Under-inflation
initialisation".
Confirm the instruction by pressing "Yes".
The new pressure settings registered are
considered to be reference values by the
system.
The under-inflation alert can only be
considered reliable if reinitialisation of
the system is done with the the four tyre
pressures correctly set.
Operating fault
The flashing and then fixed illumination of the
under-inflation warning lamp accompanied
by illumination of the "service" warning lamp
indicates a fault with the system.
In this case, under-inflation monitoring of the
tyres is no longer assured.
Have the system checked by a CITROËN
dealer or a qualified workshop.
Before doing anything to the system,
the pressures of the four tyres must be
checked and the system reinitialised.