If used by a Fire-type Pokémon, Burn Up will thaw out the user if it is frozen, and then inflict damage to the target and cause the user to lose its Fire type. Burn Up fails if the user is not Fire-type, making the move always fail after the first use until the Pokémon regains its Fire type (either by switching out, or other type changing effects such as through Protean or Reflect Type); Burn Up cannot thaw the user if the move would fail (due to the user not being Fire-type).
If the user is a pure Fire-type Pokémon, it will become typeless.
Like all type changing effects, the Pokémon's type reverts to its default types if it is switched out.
A Pokémon with Protean cannot successfully use Burn Up unless it already is Fire-type; otherwise, Burn Up will fail and Protean will not activate.