In Star Trek technobabble, a diagnostic would be performed whenever something went wrong and they didn't know why. They'd perform a diagnostic on the thingamabob or whatchamacallit that caused the problem until it coughed up the answer. Like interrogation on inanimate objects. Diagnostics are gauged by levels:

  • Level One diagnostic - routine check. They probably do it twice a week anyway.
  • Level Two diagnostic - someone might have to climb into a Jeffries Tube, or at the very least stare at a screen for an hour & push lots of buttons.
  • Level Three diagnostic - Someone's definitely getting up in that Jeffries Tube and there's gonna be overtime.
  • Level Four diagnostic - "No one's going to Ten Forward until either this ship crashes into the sun or we fix it."

I think I heard Commander Riker mention a level nine diagnostic once on an episode, but Geordi LaForge just laughed at him.