I hate being too particular but even if you are qualified on the equipment, if you aren't assigned to operate that flight, you have no business. Maybe the logbook says don't start APU, or maybe the fire detect is inop, or maybe there's a discrepancy between logbook and the flight plan, maybe the captain is discussing it now with maintenance, maybe there are tech folks on ladders working on it. It sux in that we're only trying to be helpful, but the FAA isn't very concerned with customers being too hot or too cold.