1. Easy checks first
- Battery state of charge and cranking voltage.
- Charging voltage and alternator ripple where relevant.
- Fuses, relays, grounds, loose connectors and water ingress.
- Fluid levels, oil quality, coolant level and intake duct condition.
Code scan first. Fault confirmation before parts.
A DTC tells you which circuit, system or operating condition the ECU is unhappy with. It does not automatically tell you which part to replace. This page gives the practical test order used in the app.
2. Confirm the complaint
- Record active, pending and history codes with freeze-frame.
- Note engine temperature, load, RPM and road speed when the fault set.
- Clear only after recording data, then road test if safe.
- Check if the symptom occurs cold, hot, idle, cruise or under load.
3. Use live data properly
- Fuel trims: high positive means lean correction; high negative means rich correction.
- MAF: compare airflow to engine size and load.
- Cam/crank: compare commanded and actual values.
- ABS: compare individual wheel speeds while moving slowly.
4. Prove the circuit
- Back-probe only where safe and avoid damaging terminals.
- Load-test powers and grounds; a voltmeter alone can mislead.
- Scope crank, cam, injector and ignition signals where intermittent.
- Perform wiggle tests while watching live data or scope capture.
Common diagnostic pathways
| Symptom | First checks | Next-level testing |
|---|---|---|
| Check engine light | Scan all modules, freeze-frame, battery voltage, visual inspection. | Live data, road test, circuit tests, smoke test, scope capture. |
| No start / crank no start | Battery, cranking RPM on scan tool, immobiliser light, fuel level, fuses. | Crank/cam scope, injector pulse, fuel pressure, compression. |
| Misfire | Misfire counters, plug/coil/injector swap, vacuum leaks. | Compression/leakdown, injector balance, fuel trims, ignition waveform. |
| Battery drain | Battery test, charge test, obvious lights/modules staying awake. | Parasitic draw test, voltage drop across fuses, network sleep check. |
| ABS/traction light | Wheel speed data, tyre size, hub play, visible harness damage. | Sensor power/signal scope, tone ring inspection, CAN/module checks. |
Do not replace parts just because a code names them
Example: an oxygen sensor code can be caused by a vacuum leak, fuel pressure issue, exhaust leak, misfire or wiring fault. A cam sensor code can be caused by chain stretch or VVT oil control. A transmission request code like P0700 means the TCM needs to be scanned for the real fault.