Diagnostic Methodology

The workflow begins with the best available facts about the case:

• the exact vehicle and configuration;
• the customer's concern and reported symptoms;
• DTCs, warning indicators, scan scope, and module communication status;
• the operating conditions under which the problem occurs;
• recent repairs, modifications, battery events, or other relevant history;
• measurements, inspections, or repairs already completed; and
• available tools, access limitations, and safety conditions.

Missing information should remain visible, but it should stop the workflow only when it is genuinely required to identify the vehicle, select or interpret the next check, or perform the work safely.

The absence of DTCs does not end the diagnostic process. When no relevant codes are present, the workflow can continue from the reported symptom, operating conditions, system behavior, and available evidence.

Customer reports, technician observations, measured results, service information, and diagnostic conclusions are different types of evidence. STEP preserves their source and level of certainty instead of treating them as interchangeable facts.

A DTC identifies a monitored condition. It does not, by itself, identify a failed component.

A possible cause remains a hypothesis until a relevant test, observation, or combination of consistent findings supports it. Approximate or uncertain information may still be useful, but it should remain identified as approximate or uncertain.

Vehicle-specific statements should be traceable to service information that applies to the exact vehicle and configuration.

This includes:

• connector and pin identification;
• fuse, splice, circuit, and ground locations;
• expected voltage, resistance, pressure, temperature, or waveform;
• test prerequisites and operating conditions;
• component locations;
• wiring diagrams; and
• manufacturer diagnostic procedures.

General automotive and electrical logic may be used to organize the diagnostic path, but it should not replace applicable service information or be used to invent vehicle-specific details.

STEP may maintain several working hypotheses, but the technician should receive one clear next diagnostic step at a time.

Each step should define:

• the diagnostic question being answered;
• why the check is relevant at that point in the case;
• the required tools and test conditions;
• the exact action to perform;
• the expected result or interpretation criteria;
• the information the technician should return; and
• the applicable source.

The preferred next step is the one that safely reduces the most uncertainty, separates plausible causes, and avoids repeating work that has already been completed.

When a required prerequisite is missing, the workflow should clarify it, select an equivalent test when possible, or pause at the relevant decision point. It should never invent a result.

The technician's original response and the interpreted result of each check become part of the case record.

Preserving this history:

• reduces repeated work;
• supports handoffs between technicians;
• makes changes in diagnostic direction understandable; and
• shows which conclusions were supported by which evidence.

An unperformed test is not a failed test. An unknown value is not evidence for either branch. An approximate report should not be silently converted into a precise measurement.

A diagnostic plan is a working map, not a rigid script.

A new measurement, DTC, configuration detail, repair result, or contradictory observation may require the active hypotheses and next steps to be reconsidered.

STEP should support that reassessment without rewriting the history of the case. Earlier evidence remains visible even when the interpretation of that evidence changes.

A newly discovered fault may be repaired before the entire case is complete. If the original concern remains after that repair, the completed repair is preserved as part of the case and the diagnostic process continues.

A repair or replacement recommendation should follow sufficient localization of the fault.

That may consist of:

• one decisive test that clearly isolates the failure; or
• several independent and consistent findings that support the same conclusion.

Before condemning a component, the process should distinguish the component itself from related power, ground, wiring, connector, command, communication, configuration, and mechanical conditions.

A completed repair does not automatically resolve the case. After repair, the workflow should:

• verify the repaired circuit, component, or condition;
• repeat the original symptom check;
• rescan the vehicle when applicable;
• evaluate any remaining or returning DTCs; and
• continue diagnosis if the original concern remains.

The technician remains responsible for determining whether a proposed step is appropriate and safe under the actual working conditions, performing or supervising the work, evaluating the evidence, and making the final diagnostic and repair decision.

STEP may organize the case, retrieve applicable service information, preserve evidence, propose a next step, explain why that step is relevant, and revise the diagnostic plan as new results are entered.

It does not physically inspect the vehicle, control the working environment, or independently verify that a test was performed correctly.

STEP does not replace technician training, current manufacturer service information, equipment instructions, or applicable workplace safety procedures.

Vehicle systems may involve:

• high voltage;
• fuel or hydraulic pressure;
• heat;
• moving components;
• pyrotechnic devices;
• hazardous materials;
• lifting operations; and
• other significant risks.

Before testing or repair:

• confirm the exact vehicle and configuration;
• use appropriate tools and personal protective equipment;
• follow current manufacturer procedures;
• verify all required test conditions; and
• stop when the required safety conditions, equipment, access, or training are not available.

A case should be marked resolved only when:

• the fault has been sufficiently localized and supported by evidence;
• the repair or corrective action has been recorded;
• the repaired area has been rechecked;
• the original concern is no longer present;
• remaining DTCs or symptoms have been evaluated; and
• the technician confirms that the problem has been corrected.

A completed STEP workflow provides a traceable record of the reported context, diagnostic steps, evidence, conclusions, repairs, and verification entered for that case. It improves transparency and repeatability, but it does not guarantee that every possible failure condition has been considered.