A fire truck rushed to the home of Katherine Heigl moments ago -- but TMZ has learned ... it was a steamy false alarm.
A rep for L.A. City Fire tells TMZ, a call was placed at 1:36pm reporting a possible fire -- and firefighters scrambled to the scene.
But Katherine's rep tells TMZ, the alarm was triggered by some steam inside the house -- the source is unclear. No one was hurt.
It's also unclear if Katherine was home at the time -- but her husband Josh Kelley (above) greeted firefighters at the gate ... before sending them on their way.