diagnose

verb

1. (transitive)To determine which disease is causing a sick person's signs and symptoms; to find the diagnosis.
2. (broadly)To determine the cause of a problem.

   “But in the early days of the scheme the new machines created some problems for the fitters, who found them over-complex and their faults hard to diagnose after many years' experience of small, simple steam locomotives.”

   “Mechanics use this extremely portable tool to diagnose engine faults, clear fault codes, and export data.”