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.

Di`ag*nos"tic (?), a. [Gr. able to distinguish, fr. : cf. F. diagnostique.]

Pertaining to, or furnishing, a diagnosis; indicating the nature of a disease.

© Webster 1913.

Di`ag*nos"tic, n.

The mark or symptom by which one disease is known or distinguished from others.

© Webster 1913.