Continuity mistake: In the very beginning, when Ralph is explaining to the bad guys how Fix-it-Felix Jr. works, and how he would rather be a good guy, it shows Ralph and Felix playing their game. When Ralph comes out and starts wrecking things, he puts a hole in the window to the right of the door. When Felix begins to fix things, the window now has two holes.
Continuity mistake: When Ralph and Vanellope are playing the minigame, Ralph gets up on the ramp and starts sorting the ingredients and trash. In one shot, there is a plunger headed towards the ingredients' bowl. In the next shot, the plunger is gone.
Continuity mistake: When Ralph makes the track inside Vanellope's hideout, the road is completely plain, with no obstacles. Later on in the scene, Ralph is guiding Vanellope around some sort of obstacles, which is a big group of spikes coming out of the ground. When Vanellope begins to drive on her own, the obstacles are gone.
Answer: He fixed her heart by showing her love again, but this time with a happy ending. Her back story was made so that her previous lover was eaten and killed on their wedding day. Felix is supposedly able to fix anything - not necessarily just with his special hammer. He fixed her heart with himself. He gave her a new back story.
Quantom X ★