The Impala breaks down on Sam in Some Greater Good (http://hold-onhope.livejournal.com/204319.html#cutid1) by hold_onhope. It happens in the first part of part 5.
Summary: Castiel is told to protect Sam Winchester, to teach him what he needs to know if he wants to kill Lilith. Dean is in Hell - and the angels want him there. So when Uriel raises his brother from the pit, Sam, along with the unlikely ally he finds in Castiel, makes a decision. He's going to save his brother from the angels, no matter what it takes. Season 4 AU | PG-13; violence, language| Sam, Castiel, Dean, Ruby, Bobby | 34,814
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Summary: Castiel is told to protect Sam Winchester, to teach him what he needs to know if he wants to kill Lilith.
Dean is in Hell - and the angels want him there. So when Uriel raises his brother from the pit, Sam, along with the unlikely ally he finds in Castiel, makes a decision.
He's going to save his brother from the angels, no matter what it takes.
Season 4 AU | PG-13; violence, language| Sam, Castiel, Dean, Ruby, Bobby | 34,814