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:
Key without remote control
Key (A) (or (C) in alternative), delivered in two copies when the car is not
fitted with remote control, controls the following:
❒ the ignition switch;
❒ the door and boot locks;
...
Deactivation
Turn the ignition key to STOP.
At the following starting operation (key at MAR), the sensor will not be reactivated
even is the stalk is on (B). In this event, to activate the rain sensor, you have ...
Engine
...