Turning the key to MAR the warning light on the dial turns on, but it should go off as soon as the engine is started.
If the 
warning light turns on when
the car is travelling (on certain versions together with the message on the display),
stop the engine immediately and contact a Fiat Dealership.
See also:
Headlights
ADJUSTING THE HEADLIGHT BEAM
The correct positioning of the headlight beams is very important for the comfort
and safety, not only of the person driving the vehicle but also all other road users.
...
Petrol engines
Use only unleaded petrol.
To prevent errors, the diameter of the fuel tank filler is too small to introduce
a lead petrol pump filler.
Use petrol with a rated octane number (R.O.N.) not lower than ...
Incomplete door locking
(red)
On certain versions the warning light turns on when one or more doors or the
tailgate are not properly shut.
On certain versions the dedicated message is displayed; symbols
indicate resp ...