Why Is Your Check Engine Light On (+How to Diagnose)

What Is a Check Engine Light?

The Check Engine Light (CEL) is a part of your vehicle’s on-board diagnostic system.  It’s a general warning light that alerts you when one or more of your car’s systems needs repair or immediate attention.

How does it work?
Modern vehicles have sensors that monitor components for real-time data such as temperature, pressure, and fluid levels. These sensors relay the data to the Engine Control Unit (ECU). If the ECU receives abnormal readings due to a malfunctioning part, it’ll activate the Check Engine Light on the dashboard to alert the driver. 

Depending on your car’s make and model or the severity of the issue, CEL can appear as a yellow, orange, or red engine icon accompanied by the text “check engine.”

10 Possible Causes Why Your Check Engine Light is On

Some common reasons for an illuminated CEL are: 

1. Loose Gas Cap

The gas cap seals the engine’s fuel system to maintain proper pressure and prevent fuel from evaporating. If the gas cap is loose or broken, it disrupts the pressure balance in the Evaporative Emission Control System (EVAP), triggering the CEL.

2. Faulty Oxygen Sensor

The O2 sensor monitors the amount of oxygen in the exhaust gases and sends this data to the ECU. But if your vehicle has a faulty oxygen sensor, it’ll prompt the ECU to activate a flashing Check Engine Light.

3. Weak Car Battery

Your battery powers the car’s electrical systems and provides the necessary charge to start the engine. When the battery weakens, it may struggle to supply adequate power to the vehicle’s sensors and electrical components, potentially triggering the CEL.

4. Engine Misfire

The engine ignites a precise air-fuel mixture to run the vehicle. Misfires, caused by worn spark plugs, clogged fuel injectors, failing ignition coils, or vacuum leaks, disrupt the combustion process, causing a flashing Check Engine Light. 

5. Faulty Mass Airflow Sensor (MAF)

The mass airflow sensor measures the air entering the engine. But if it has faulty wiring, the sensor can send incorrect information about the airflow. The inaccurate data can cause the ECU to miscalculate the fuel needed for proper combustion, leading to an imbalanced air-fuel ratio. This can affect engine performance, impact fuel economy, and increase emissions, triggering the CEL.

6. Catalytic Converter Issues

The catalytic converter reduces harmful emissions by converting exhaust gases into less toxic substances. When this catalytic converter malfunctions, it increases emissions, triggering the ECU to activate the CEL.

7. Worn Timing Belt

The timing belt ensures the engine’s camshaft and crankshaft work in perfect synchronization, maintaining proper valve timing for efficient combustion. However, a worn timing belt can disrupt this engine timing, leading to a Check Engine Light issue.

8. Damaged Exhaust Pipe

The exhaust pipe typically channels harmful exhaust fumes away from the engine and reduces combustion-related noise. If the pipe is damaged, it can increase emissions and reduce fuel efficiency. This may also trigger the Check Engine Light if the issue affects emissions or engine performance significantly.

9. Low Fluid Levels

Oil lubricates engine components, while coolant regulates the engine’s temperature. Insufficient oil and coolant levels can lead to overheating, reduced fuel economy, and system inefficiency, prompting the CEL to activate.

10. Cooling System Issues

The vehicle’s cooling system maintains optimal engine temperature. When components like the thermostat or radiator fail, it can lead to overheating, which may activate the CEL.

Is It Safe to Drive With the Check Engine Light On?

No, driving with an illuminated Check Engine Light isn’t recommended. It can signal various issues, ranging from a minor issue, like a loose gas cap, to a more serious one such as malfunctioning oxygen sensors.

While a constantly illuminated Check Engine Light may seem harmless, it’s crucial to address it promptly to prevent costly repairs later.

However, if the Check Engine Light is flashing, it’s an emergency warning. It usually points to a serious problem, such as a faulty spark plug or worn ignition coil, requiring immediate attention.  

Remember, driving with a flashing CEL can cause an engine misfire and compromise vehicle safety.

To prevent a minor problem from becoming a dangerous one, schedule a comprehensive engine diagnostic service or visit an auto repair shop as soon as possible.

How Do You Diagnose and Reset a Check Engine Light?

When the CEL turns on, the ECU stores a diagnostic trouble code (DTC), providing a clue about the underlying issue. 

Each Check Engine Light diagnostic code begins with a letter indicating the system affected (e.g., “P” for Powertrain), followed by numbers with more specific details. However, a diagnostic code is just a starting point — you’ll need further examination by a certified technician to pinpoint the exact problem.

NOTE: A minor issue like a loose gas cap won’t necessarily need a check engine light service. However, serious problems, such as a failing catalytic converter or transmission troubles, require professional attention from a certified technician.

Here’s how a mechanic will perform a Check Engine Light diagnostic service:

1. Use the OBD-II scanner to retrieve each diagnostic trouble code (DTC), such as P0128. Based on the stored engine light codes, they’ll check different car components. 
2. Ensure the gas cap is secure.
3. Examine the vacuum hoses for visible leaks or loose connections.
4. Check the battery terminals, clean any corrosion, and ensure a proper electrical connection.
5. Inspect engine components, including spark plugs, ignition coil, fuel injectors, and timing belts, for signs of damage.

After performing vehicle maintenance and repairs, the mechanic will reconnect the OBD-II scanner to clear existing engine light codes and reset the CEL.

How Much Does a Check Engine Light Diagnosis Cost?

An auto repair shop typically charges between $50 and $200 for a Check Engine Light diagnosis. However, the final price can vary based on your vehicle’s model, local labor rates, and applicable taxes.

During the diagnostic service, the mechanic might recommend other necessary repairs, which will incur additional costs. For example, an O2 sensor replacement will cost you an extra $360 to $520.

Is the Check Engine Light and “Service Engine Soon” Light the Same?

No, the Check Engine Light and the “Service Engine Soon” warning light aren’t the same. 

A Check Engine Light points to urgent problems with your engine or related systems that require immediate attention. 

Whereas the “Service Engine Soon” warning light serves as a reminder for scheduled maintenance tasks, such as oil changes or air filter replacements. While the SES light doesn’t signal an immediate concern, ignoring it can lead to missed maintenance and potentially serious issues.

How to Prevent the Check Engine Light From Activating

Use these measures to avoid Check Engine Light issue in the future: 

• Follow vehicle maintenance routine for oil changes, brake service, spark plug replacements, and other essential engine performance checks.

• Avoid aggressive driving, as it can strain your engine.

• Choose reputable gas stations and high-quality oil for fuel efficiency and to prevent engine issues that can trigger the CEL.

• Pay attention to all warning lights, not just the CEL, to catch potential issues early.

• Don’t delay fixing a minor problem, such as a loose gas cap; it can lead to more expensive repairs later.