every game has the same types and weaknesses. as usual psychic is weak to bug and dark type Pokemon in all games, not just Pokemon platinum.it's also weak to ghost
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Dark Pokemon are Psychic Pokemon's weakness.
flying type psychic type
fighting type pokemons weakness are psychic and flying moves steel is weak aageinst water fire fighting and ground
EDIT: and dark. physical
Mostly fire element's weaknesses are Water and Ice.....BUT...on some fire type Pokemon's Weaknesses also include Psychic.....Happy Training you Pokemons!