There are several possible reasons why the check engine light (CEL) might be illuminated in your 2018 Honda Accord. It's important to approach this methodically to effectively diagnose and rectify the issue. Below are some potential causes and steps to consider:

 
Check the Gas Cap
 

• The gas cap is a common culprit for triggering the check engine light.
• Ensure that it's tightly secured. A loose or missing gas cap can cause fuel vapors to leak, which may trigger the engine light.
• If the gas cap is damaged, consider replacing it.

 
Inspect Air Filter and Intake System
 

• A clogged or dirty air filter can reduce the engine's performance and may turn on the check engine light.
• Check the air filter to see if it needs cleaning or replacement.
• Inspect the intake system for any obstructions or leaks.

 
Evaluate the Oxygen Sensors
 

• Oxygen sensors monitor the level of oxygen in the exhaust. Failure or degradation of these sensors can trigger the CEL.
• If the oxygen sensor is faulty, it might need replacement to improve fuel economy and reduce emissions.

 
Examine the Catalytic Converter
 

• A failing catalytic converter can cause your check engine light to illuminate.
• Issues with the converter can often be detected through symptoms like reduced performance or unusual exhaust smells.
• Catalytic converter problems may require professional attention and repair or replacement.

 
Check Engine Oil and Fluids
 

• Ensure the engine oil levels are adequate and that the oil is clean. Low or dirty oil can affect engine performance and trigger the CEL.
• Check other vital fluids such as coolant and transmission fluid, as imbalances can also cause the light to come on.

 
Investigate Spark Plugs and Ignition Coils
 

• Faulty spark plugs or ignition coils can lead to misfires, reduced fuel efficiency, and activation of the CEL.
• Inspect spark plugs for wear and replace if necessary. Ignition coils might need testing and possible replacement if they are not functioning properly.

 
Utilize an OBD-II Scanner
 

• An OBD-II scanner can read the diagnostic trouble codes (DTCs) stored in your car's computer.
• The codes can provide insight into what might be triggering the CEL and guide you or a professional mechanic towards a solution.

 
Consult a Professional Mechanic
 

• If the cause of the check engine light is not immediately apparent, it may be time to consult a professional.
• A certified mechanic can perform a more thorough diagnosis and offer the most effective repair plan.

Remember that ignoring the check engine light can lead to more serious issues over time, so it is best to address it promptly.