1. Gather Information (Customer Interview)
Diagnostics start before touching the car.
A technician asks questions like:
- What symptoms are happening?
- When did the issue start?
- Does it occur all the time or only under certain conditions?
- Were any repairs done recently?
- Are warning lights on?
👉 This step helps narrow the system involved and saves time later.
2. Verify the Concern
The technician attempts to reproduce the problem:
- Test drive the vehicle
- Observe noises, vibrations, smells, or drivability issues
- Confirm dashboard warning lights
If the problem can’t be verified, it’s much harder to diagnose accurately.
3. Perform a Preliminary Inspection
This is a quick but critical visual and basic check:
- Fluid levels and condition
- Obvious leaks
- Loose or damaged wiring
- Broken hoses, belts, or connectors
- Battery condition and connections
Many faults are found right here.
4. Scan for Diagnostic Trouble Codes (DTCs)
Using a scan tool, the technician:
- Retrieves trouble codes
- Reviews freeze-frame data (engine conditions when the fault occurred)
- Checks pending or history codes
⚠️ Important:
A code tells you where the system noticed a problem, not which part to replace.
5. Research and Analyze Data
Technicians consult:
- Factory service information
- Wiring diagrams
- Technical Service Bulletins (TSBs)
- Known failure patterns
They also look at live data from sensors to see if readings make sense.
6. Pinpoint Testing (Root Cause Diagnosis)
This is the most important step.
Depending on the issue, testing may include:
- Voltage drop tests
- Resistance and continuity checks
- Sensor signal analysis
- Smoke testing (for vacuum/EVAP leaks)
- Fuel pressure testing
- Mechanical tests (compression, leak-down)
🔧 Parts are tested, not guessed.
7. Identify the Root Cause
The technician confirms:
- What failed
- Why it failed (wear, contamination, wiring fault, mechanical damage, etc.)
- Whether related components are affected
This prevents repeat failures.
8. Perform the Repair
Only after diagnosis is confirmed:
- The faulty component is repaired or replaced
- Related issues are addressed
- Software updates or calibrations are performed if needed
9. Verification & Post-Repair Testing
After repair:
- Codes are cleared
- The vehicle is test-driven
- Live data is rechecked
- The original complaint is confirmed resolved
A proper repair is verified, not assumed.
10. Documentation & Customer Explanation
Professional diagnostics includes:
- Documenting test results
- Explaining the problem in plain language
- Showing why the repair was necessary
This builds trust and transparency.
Key Principles of Proper Diagnostics
✔Systematic, not rushed
✔Based on testing, not parts-swapping
✔Uses data and service information
✔Verifies the fix