Catalyzes three successive oxidations of the 4-methyl group of ent-kaurene giving kaurenoic acid, a key step in gibberellin (GA) biosynthesis. ent-kaur-16-ene + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = ent-kaur-16-en-19-oate + 4 H(+) + 4 H2O + 3 oxidized [NADPH--hemoprotein reductase] Plant hormone biosynthesis; gibberellin biosynthesis. This protein is poorly expressed and has no detectable ent-kaurene oxidase activity in this species. Consequently, this species does not produce gibberellins. Belongs to the cytochrome P450 family.