1. Understand the “Check Engine” Light

• When the "Check Engine" light illuminates, it indicates that the car's onboard diagnostics (OBD) system has detected an issue with the vehicle's engine, emission system, or another integral component.
• Common reasons for the light include a loose gas cap, faulty oxygen sensor, or more serious engine problems.

2. Secure the Gas Cap

• One of the easiest and most frequent fixes for the "Check Engine" light is to ensure that the gas cap is properly tightened. A loose or damaged gas cap can cause fuel vapors to leak, leading to an error signal.
• Remove the gas cap, inspect it for any damage, and re-tighten it until it clicks.
• Start the car and see if the light turns off. If the light persists, continue to the next step.

3. Use an OBD-II Scanner

• Purchase or borrow an OBD-II scanner. This tool connects to your vehicle and reads diagnostic trouble codes (DTCs) stored in the car's computer system.
• Locate the OBD-II port in your 2018 Ford Mustang EcoBoost, which is typically under the dashboard on the driver's side.
• Plug in the scanner, turn on the ignition (do not start the engine), and follow the scanner’s instructions to retrieve the codes.
• Note the codes that appear. These codes will tell you what's wrong and where to start troubleshooting.

4. Diagnose the Problem

• Use the codes retrieved from the OBD-II scanner to diagnose the problem. Common codes might include issues with the oxygen sensor, catalytic converter, ignition coils, or spark plugs.
• Refer to your vehicle’s service manual or search for the specific code explanations online to understand the potential causes and solutions.

5. Address the Issue

• Replace or repair the component that has triggered the "Check Engine" light. If the issue is something simple like a spark plug or oxygen sensor, you may be able to handle it yourself with basic tools.
• If the problem is more complicated or you're not comfortable performing the repair, take your Mustang to a certified mechanic.

6. Clear the Codes

• After addressing the issue, you’ll need to clear the codes from the OBD-II system to reset the "Check Engine" light.
• Use the OBD-II scanner to clear the codes by following the device's instructions.
• Drive the car to ensure that the issue is resolved and that the light does not come back on.

7. Prevent Future Problems

• Regular maintenance is key to preventing the "Check Engine" light from turning on. Stick to your 2018 Ford Mustang EcoBoost’s maintenance schedule, including oil changes, air filter replacements, and other routine checks.
• Pay attention to any unusual noises or changes in performance, and address them promptly.