CITROËN DS4 Owner's Handbook Manual page 214

Stop & Start
The Stop & Start system puts the engine temporarily into standby - STOP mode - during stops in the traffic (red lights, traffic jams, or other...).
The engine restarts automatically - START mode - as soon as you want to move off. The restart takes place instantly, quickly and silently.
Perfect for urban use, the Stop & Start system reduces fuel consumption and exhaust emissions as well as the noise level when stationary.
Operation
Going into engine STOP mode
The "ECO" warning lamp comes
on in the instrument panel and the
engine goes into standby:
- with a manual gearbox , at speeds below
12 mph (20 km/h), when you put the gear
lever into neutral and release the clutch
pedal,
- with the 6-speed electronic gearbox
system , at a speed below 5 mph (8 km/h),
when you press the brake pedal or put the
gear lever into position N .
If your vehicle is fitted with the
system, a time counter calculates
the sum of the periods in STOP
mode during a journey. It resets
itself to zero every time the ignition
is switched on with the key.
With the electronic gearbox system,
for your comfort during parking
maoeuvres, STOP mode is not
available for a few seconds after
coming out of reverse gear.
STOP mode does not affect the
functionality of the vehicle, such as
braking, power steering, for example.
Never refuel with the engine in STOP
mode; you must switch off the ignition
with the key.
Special cases: STOP mode
unavailable
STOP mode is not invoked when:
- the driver's door is open,
- the driver's seat belt is not fastened,
- the vehicle has not exceeded 6 mph
(10 km/h) since the last engine start
using the key,
- the electric parking brake is applied or
being applied,
- the engine is needed to maintain a
comfortable temperature in the passenger
compartment,
- demisting is active,
- some special conditions (battery charge,
engine temperature, braking assistance,
ambient temperature...) where the engine is
needed to assure control of a system.
In this case, the "ECO" warning lamp
flashes for a few seconds, then goes
off.
This operation is perfectly normal.