Diagnosis, Testing and Fault Rectification
Diagnosis, Testing and Fault Rectification - what the exam tests
A structured diagnostic process, use of DTCs and scan data, multimeter and oscilloscope technique, voltage-drop and parasitic-draw testing, and correct verification of the repair.
The method
- Verify and record the complaint; check service history and obvious causes such as fuses, connectors and battery state.
- Retrieve DTCs and freeze-frame data. A code names a circuit or system, not a part to replace.
- Study the wiring diagram and identify feeds, grounds and control paths.
- Test at connectors under load and split the circuit to isolate the fault.
- Repair, clear codes, then verify under the original conditions.
Key techniques
- Voltage-drop testing under load finds resistance an ohmmeter misses.
- Current clamp for parasitic draw: typically under 20–50 mA once modules sleep.
- Oscilloscope for injector, ignition and CAN-bus signals.
- Wiggle test, plus heat and cold tests, for intermittent faults.
Worked example
Slow crank, battery good at 12.6 V. During cranking, measure from the battery negative post to the engine block: 0.9 V against a limit under 0.2 V. That 0.9 V is lost in the ground strap, leaving less voltage for the starter. Clean and refit the strap; the drop falls to 0.05 V and cranking returns to normal. Reasoning: voltage drop is measured while current flows, exposing what a no-load continuity check would call fine.
Traps that cost marks
- Replacing parts on the strength of a DTC alone.
- Using an ohmmeter on a live circuit.
- Checking draw with a door open or a module still awake.
- Skipping the re-test after the repair.
- Chasing an intermittent fault by guesswork instead of using freeze-frame data.