You'd get a fault code scan done, the warning light has come on so it's stored a code.
It'd be anything at this point, especially if you haven't felt any issue during driving.
If you haven't got a scanning tool, you need a laptop,
Multiecuscan software cost ~£50, then a few cables specifically for Fiat/Alfa/Lancia, there's plenty on eBay, most important one is Elm327, or someone from the forum might be near you who's got one.
Otherwise you gotta visit a reputable garage near you, Italian cars specialist even better, try to look for at least 4star and 100+ reviews.