Automotive Diagnostics
Basic Process and Basic Mistakes

Automotive Diagnostics Process & Common Mistakes

When it comes to diagnosing faults, the automotive diagnostics process is both an art and a science. From strange engine noises to dashboard warning lights, every symptom is a clue. But good diagnostics requires more than guesswork—it demands a logical, structured approach.

In this article, we’ll walk through the basic diagnostic steps, highlight the common pitfalls even experienced technicians make, and finish with a real-world example to bring it all together.

The Basic Diagnostic Process

Diagnosing a car fault should follow a clear step-by-step structure:

1. Collect fault information
   What are the symptoms? Do they happen at certain speeds, temperatures, or times?

2. Confirm the fault
   Try to recreate or observe the problem—this might involve a road test, checking lights, or inspecting components.

3. Understand the system
   Know how the system operates and how it interacts with others. Research before testing.

4. Collect more data
   Use scan tools, visual inspections, and manual measurements to gather detailed information.

5. Analyse and form a theory
   Consider all causes and weigh the evidence. What fits best?

6. Test to prove or disprove your theory
   Develop a test plan. If your theory is wrong, go back to step four.

7. Perform the repair

8. Verify the fix
   Always confirm the issue is resolved after the repair.

Common Mistakes in Diagnostics

Many techs skip steps—and it costs them time and accuracy. Here’s where people often go wrong:

• Jumping to conclusions
  Skipping analysis leads to the wrong repairs. Don’t go from symptom to solution without testing.

• Assuming same symptoms = same cause
  Prior cases help guide you, but each diagnosis is unique. Don’t shortcut the process.

• Misusing DTCs
  A fault code is a starting point, not a final answer. For example, P0335 (crankshaft sensor fault) could be a wiring issue, ECU fault, or sensor problem. All must be considered.

• Trusting opinions over facts
  Always ask: Does this person understand the system? Can they offer real testing ideas, not just guesses?

• Ignoring all the evidence
  Fixating on one clue while missing others can lead you far off track. Use every symptom and test available.

Example: Another shop diagnosed a diesel engine with low compression in cylinder 4 and recommended engine replacement. But it idled smoothly, which wouldn’t happen with a bad piston. Our team found a sticking throttle plate causing low air and false compression readings. A simple clean-up solved the issue.

Worked Example: Diagnosing a Toyota Prius

Here’s how the process played out on a 2006 Prius with an intermittent inverter fault:

Step 1 – Collecting info:
Customer reported power loss on the motorway and warning lights.

Step 2 – Confirming the fault:
No issue during a test drive, but fault code P0A93 (inverter cooling) was logged.

Step 3 – Understanding the system:
Visual check confirmed the inverter uses a radiator, coolant, and electric pump.

Step 4 – More info:
Coolant levels OK. No visible leaks. Pump didn’t respond to scan tool control.

Step 5 – Theory:
At high loads, pump failure causes inverter to overheat—fits the symptoms.

Step 6 – Testing:

• Power and ground were present at the pump plug.

• High resistance across pump terminals indicated internal failure.

• External power supply couldn’t activate the pump either.

Step 7 – Repair:
Replaced the inverter coolant pump.

Step 8 – Verification:
New pump vibrated and circulated coolant. Fault fixed.

Final Thoughts

Following a structured automotive diagnostics process saves time, money, and frustration. Avoid jumping to conclusions, use all available evidence, and rely on facts—not assumptions. Whether you’re a DIY enthusiast or a seasoned tech, a systematic approach is your best diagnostic tool.

If you’re ever in doubt, seek expert advice—and remember, good diagnostics is about solving problems, not guessing.