The first step in diagnosing the check engine light is to retrieve the diagnostic trouble codes (DTCs) stored in the vehicle's onboard computer. This is done using an OBD-II scanner or code reader.

Connect the OBD-II scanner to the vehicle's OBD-II port, typically located under the dashboard near the driver's seat.

Use the OBD-II scanner to read the diagnostic trouble codes stored in the vehicle's computer. Each code corresponds to a specific issue or malfunction.

Interpret the DTCs to understand the nature of the problem. DTCs are alphanumeric codes, and their meaning can be found in the vehicle's service manual or through online resources.

Some codes indicate minor issues, while others may point to more critical problems. Assess the severity of the issue based on the DTCs.

Perform a visual inspection of the engine compartment to look for obvious issues such as loose wires, disconnected hoses, or visible leaks.

A loose or faulty fuel cap can trigger the check engine light. Ensure that the fuel cap is tightened securely.

In some cases, certain codes may be triggered by specific driving conditions. A test drive may be necessary to replicate the conditions that trigger the code.

Depending on the nature of the issue, additional diagnostic tests may be needed. This can include testing sensors, checking electrical components, or performing more advanced tests with specialized equipment.

Refer to the vehicle's repair manuals or technical service bulletins (TSBs) for guidance on specific issues and recommended diagnostic procedures.

Once the issue is identified, take the necessary steps to address it. This may involve replacing a faulty sensor, repairing a wiring issue, or addressing a component that is not functioning correctly.

After addressing the issue, use the OBD-II scanner to clear the diagnostic trouble codes. This resets the check engine light. Note that clearing codes without fixing the underlying problem may cause the light to come back on.

Retest the vehicle to confirm that the issue has been resolved. Ensure that the check engine light remains off.