External diagnostics would cover a number of different areas in the transmission repair facility.
Visually Check on Transmission Assembly for Fluid Leaks
Visual Check on Transmission Case for Cracks or Flawed Castings etc.
Check for Broken or Deteriorated Engine and Transmission Mounts
Visual Check on Wiring Harness Damage (Broken, Melted etc.)
Visual Check on any Drive Line Component that Would Affect Proper
Pressure Testing Automatic Transmissions/Transaxles for Hydraulic System Integrity