We here in the USA can't see your vehicle on FORD website - since the Fiesta was "region specific" Italy is not listed nor is the owner's manuals for those cars even available to view to even help you.
We will need some form of help or information from you before we can even help diagnose this.
But to help - I use a basic standard approach to why something works when it does, but not all the time when you need it.
That relates to several conditions - and since that is a 2009 - the basic simple stuff like clean connections from the Battery to ground, Starter, if you can get past the wiring harness - you'll need to inspect the solenoid on it and the Alternator - and it's wiring - to make sure the Battery is getting a full (or as much as it can) charge to help the system shut down when you turn off the engine.
The Battery minder control - that keeps the battery from going dead because of a power drain - may be causing this condition where the system does not reset properly- because the battery is weak from lack of charge or the wiring is corroded to a point that it becomes a dropping resistor fooling the system into thinking something is left on so it aborts any shut down processes and since you didn't say too much about how you clear the condition - this is all I have to speculate on.
This boils down to, loose, corroded or broken wiring onto...
The fuse box as well as the body control module - any wiring harness problems - loose connectors - plugged cabin filter?
Radiator - check the coolant tank - it's not empty - nor leaking...right?
How long ago was the coolant changed, and how many miles does this car have on the odometer?
Thermostat - any service done to the engine? Speaking of the engine type, does it have the Duratorq or Duratec (Zetec) motor or something more exotic that Fiestas used back then? (Gas or Diesel?)
Since this is climate control - it may have problems with the actuators, or something jammed the rotating drum used to open ports to change the vents that blow air from the motor inside the drum - onto the fuses controlling this unit - which brings up the owner's manual. Which I can't find on our site Ford lets us Americans use - so any help from you to provide more information including the condition of the engine compartment in pictures or otherwise can help us to help you.