Symptom-First Diagnostic Workflows

Symptom-first diagnosis

A symptom search ("why does my car crank but not start?") is often the first question a technician asks — before any code is read. These pages target symptom-first searches and apply the MechanIQ evidence-driven framework, not "common causes" listicles.

The diagnostic framework

Every symptom page follows the same sequence:

1. Clarify the complaint — the exact symptom and its conditions
2. Confirm the operating condition — cold/hot, load, wet/dry, after what
3. Retrieve DTCs — all modules, not just the engine
4. Inspect freeze-frame and live data — the conditions when it last failed
5. Identify the affected system — narrow from symptom to system
6. Form a differential — rank hypotheses by the evidence
7. Select the decisive test — the test that most changes probability
8. Update the hypothesis — from the result
9. Confirm the cause — strong, confirmatory evidence
10. Repair
11. Verify the original symptom — under the condition that failed

Why not "common causes"

A list of "common causes for rough idle" invites parts-guessing. The framework instead uses the symptom to narrow the system, then tests to confirm — so the cause is found from evidence, not from a list.