Fault Diagnosis and Troubleshooting
The Fault Diagnosis and Troubleshooting domain tests a repeatable method rather than guessing. Marks come from asking the right question, choosing the cheapest discriminating test, and proving the repair.
A repeatable method
- Interview: what happened, when, after a drop, water or update? Which functions fail?
- Inspect: body damage, screws, moisture indicators, corrosion, port lint, battery swelling.
- Verify the symptom yourself and reproduce it.
- Isolate with half-splitting and known-good substitution: another battery, charger, cable, SIM, known-good flex.
- Repair, then retest the original symptom and the functions your work touched.
- Document the fault, parts used and result on the job sheet.
Symptom patterns
- No charge: cable and charger, port contamination or damage, charging IC, battery connector.
- No display: backlight, display connector, cracked flex, PMIC rail.
- No network: antenna contact, coaxial cable, SIM reader, power amplifier.
- Water damage: corrosion under shields, so clean ultrasonically before powering.
Worked example
A customer says the phone does not charge. You first try a known-good charger and cable, then inspect and clean the port; still no charge. Measuring VBUS at the port shows 5 V, and the charging IC output at the battery connector stays at 3.6 V with no current. Reasoning: power reaches the board but the charger is not regulating, so the charging IC or its feedback path is suspect, not the port. You replace the IC and confirm current rises.
Traps that cost marks
- Shotgun replacement of parts without a test.
- Not asking about a recent drop or liquid.
- Skipping the retest, so the same fault returns.
- Writing no record of the IMEI and accessories.