please help(fusion)

Hi
i want to fuse two face (face1 and face2)
so i write this code :

shape1=BRepAlgoAPI_Fuse(face2,face1);
BRepBuilderAPI_MakeWire M;
for (ex1.Init(shape1,TopAbs_EDGE);ex1.More();ex1.Next())
{
E2= TopoDS::Edge(ex1.Current());
M.Add(E2);
}
face3 = BRepBuilderAPI_MakeFace(M);

the problem that sametimes gives a bad fusion
i'm a beginer in OC, so please can you help me to resolve this problem

thank you

waw