Why does my 2017 Ford fiesta smell like rotten eggs?

Why does my 2017 Ford fiesta smell like rotten eggs?

If the catalytic converter has been removed, that is why. Otherwise it is running rich at least some of the time.

Under normal conditions sulfur compounds in the gasoline are oxidized in the catalytic converter to sulfur dioxide that burns the eyes and nose in large amounts but normally is less than that. Without a working catalytic converter, or if there's not enough oxygen to oxidize the sulfur it combines with the hydrogen to produce hydrogen sulfide, the smell you are experiencing.

That smell usually is from a catalytic converter that has gone bad, BUT it's not common for a one year old vehicle to have that issue… If the vehicle is under warranty, then have a dealer look at it to determine what the odor is

That's the converter working.

That's Hydrogen Sulphide from the catalytic convertor.