The Grim Adventures of Billy and Mandy (2001) : 4x3

As Billy is tired of all the rules in his house, he cannot resist the urge to try his father's new motorcycle. The motorcycle breaks down, and Billy asks Grim to repair it. Grim's scythe turns the motorcycle into a wild, supernatural vehicle, which transforms Billy's parents and everyone around it into undead creatures and motorized monsters. Mandy makes Billy tell the truth about breaking the motorcycle in the first place, at which point his parents and everyone else return to normal.