Which of the following steps is part of fault isolation per CSOSS?

• A. Identify fault and log it.
• B. Verify symptom and observe.
• C. Isolate probable cause and plan next steps.
• D. Substitute or bypass if safe, re-verify, and document.

Answer: (D)

Fault isolation means proving exactly where the fault lies by testing the system in a controlled way while keeping safety in mind. Substituting or bypassing the suspected faulty path or component, when it’s safe to do so, directly tests whether that part is causing the problem. If the system behavior returns to normal with the bypass, you’ve gained a strong indication that the bypassed element is the fault source; if the symptom remains, the fault lies elsewhere. After performing the bypass, re-verify the system to confirm the result and ensure the symptom is truly resolved or persists as expected. Documenting what you did and what you observed is essential so others can understand the diagnostic steps taken and the rationale for the next actions. This combination—substitute/bypass to test, re-verify the outcome, and record the results—is the concrete, repeatable action set CSOSS uses to isolate faults.