What is the purpose of substitution tests in fault isolation?

• A. To quickly replace the entire system.
• B. To verify the color coding on cables.
• C. To test power levels.
• D. Replace components one by one with known-good parts or perform substitution tests.

Answer: (D)

Substitution tests are about quickly identifying which part is causing a fault by swapping in known-good replacements and seeing if the problem follows the part. In practice, you replace the suspected component one by one with a working equivalent and observe whether the symptom goes away. If the fault is resolved after swapping a part, that component is likely faulty; if the problem remains, the issue lies elsewhere in the circuit or in how parts interact. This method is efficient because it directly tests a component’s health rather than guessing from symptoms alone. Replacing the entire system or just checking cable colors or power levels won’t isolate which single part is bad, whereas substitution targets the actual faulty element by using known-good parts.