Is expiation (kaffarah) obligatory, or do they incur sin, for their car breaking down on the way to Umrah and being forced to return?

If the pilgrim returns before entering the state of ihram (definition), there is no sin or fidyah (definition) (expiation). However, if they return after entering the state of ihram, they must both return to Makkah to complete the 'Umrah (definition) with the previous ihram. They must also avoid the prohibitions of ihram, and if they commit any of them, they are subject to the corresponding fidyah, hady (sacrificial animal), or qada' (making up for it).