During fault isolation, when is it appropriate to substitute or bypass a suspected faulty component?

• A. Only after the fault is confirmed and there is a safe, documented path for bypass.
• B. As soon as a fault is suspected, regardless of safety.
• C. After full system reboot and re-check.
• D. If safe and permitted by procedure, substitute or bypass, then re-verify and document.

Answer: (D)

The main idea is to use substitution or bypass only when it’s safe and explicitly allowed by the procedure, and then to re-check the system and document everything. When you suspect a component is faulty, performing a controlled bypass or substitute lets you test whether the fault follows that path or lies elsewhere, but doing so without procedure approval or safety safeguards can create hazards or hide other problems. If the substitution or bypass is permitted, you re-verify the system to see if the fault condition disappears or changes, which helps pinpoint the faulty area. Finally, you document what was done, the results, and why the action was taken so maintenance records are accurate and future troubleshooting is informed.