Fault Diagnosis and Troubleshooting
What this domain tests
Fault Diagnosis and Troubleshooting tests a logical path from symptom to cause: interview, visual inspection, power/current measurement, split-half testing, substitution and verification.
Core rules
- Record the complaint in the customer's words and the phone's history: drop, water, repair, update.
- Inspect for corrosion, burn marks, cracked shields, bent pins and missing components before powering.
- Use a current-limited bench supply at the battery voltage. Note current draw: near zero suggests an open path; high current with heat suggests a short; a cycling draw suggests boot attempts.
- Split the circuit: confirm input power, then PMIC rails, then the affected block. Substitute only known-good parts.
- Verify the repair by testing all functions, not just the reported one.
Worked example
A phone shows no power. The bench supply at 4 V draws 0.00 A even after pressing power. That means current is not flowing, so start at the charging port and battery connector: check for broken solder or a disconnected flex. If input voltage reaches the PMIC but no rails come up, check the power key and PMIC enable. By contrast, a 0.6 A draw with a hot area near the PMIC indicates a short; use thermal inspection or controlled voltage injection only within safe limits. The current reading directs the search instead of guessing.
Traps
Replacing parts without measuring; testing with a discharged battery; ignoring water damage corrosion under shields; misreading a charger's current as phone current; using unlimited current and causing more damage.