Diagnostic Equipment, Fault Finding and Testing Procedures
What the exam tests
A structured diagnostic process, scan tool use, scope and meter testing, and verification.
Core
- Verify the complaint, gather symptoms and conditions, check for technical bulletins, then read DTCs and freeze-frame data before clearing anything.
- A DTC identifies a circuit or system, not automatically the component. Use live data and pinpoint tests.
- Scan tools show serial data, actuator tests, adaptations and coding. OBD-II generic data may be limited compared with manufacturer data.
- A scope shows waveform shape, amplitude, frequency and duty cycle. Set the time base and voltage scale before probing; use a suitable attenuator for injectors and ignition.
- Voltage-drop testing finds high-resistance joints under load. Resistance testing with no load can pass a cable that fails under current.
- Parasitic draw: connect a meter in series, wait for sleep, then measure voltage drop across fuses to identify the circuit.
Worked example
A battery drains overnight. After sleep, current is 420 mA. Instead of pulling every fuse, measure millivolts across each fuse with the meter in DC mV. A fuse with a measurable drop is carrying current. Look up the drop on a fuse-resistance chart for that rating: a 10 A fuse at 0.5 mV is small, but 5 mV may indicate several hundred milliamps. Remove that circuit's fuse and confirm the current falls. Then inspect the module on that circuit.
Traps
Treating a DTC as a parts list; clearing codes before recording freeze frame; using the wrong meter range; resistance-testing a loaded circuit; failing to verify the repair by repeating the original test.