Home FAQ

What is the common reason the check engine light comes on in most vehicles?

The check engine light, also known as the malfunction indicator light (MIL), is a warning signal that illuminates on the dashboard of a vehicle when the onboard computer detects an issue with the engine or other components. In most cases, the check engine light comes on due to a problem that needs to be addressed, though the specific reason can vary.

Reasons the Check Engine Light Illuminates

Faulty Oxygen Sensor: The oxygen sensors in a vehicle's exhaust system monitor the air-fuel mixture. If one of these sensors is not functioning properly, it can trigger the check engine light.

Loose or Faulty Gas Cap: A loose, damaged, or missing gas cap can cause the check engine light to turn on, as it disrupts the vehicle's fuel system and allows evaporative emissions to escape.

Catalytic Converter Issues: The catalytic converter is responsible for reducing harmful emissions. If it becomes clogged or damaged, the check engine light may illuminate.

Malfunctioning Spark Plugs or Ignition Coils: Worn or faulty spark plugs or ignition coils can cause misfiring in the engine, leading to the check engine light turning on.

Problems with the Mass Airflow Sensor: The mass airflow sensor measures the amount of air entering the engine. If it is not functioning correctly, the check engine light may activate.

These are some of the most common reasons the check engine light may come on in a vehicle. It's important to have the issue diagnosed and addressed promptly to avoid further damage or decreased fuel efficiency.

Diagnosing and Addressing Check Engine Light Issues

When the check engine light illuminates, it's recommended to have the vehicle's onboard diagnostic system scanned to determine the specific problem. This can be done at most auto parts stores or by a professional mechanic. Once the issue is identified, the necessary repairs can be made to turn off the check engine light and ensure the vehicle is running properly.

Does AutoZone check engine light for free?

Yes, AutoZone offers a free Check Engine Light service called Fix Finder, which uses a scanner to retrieve trouble codes from your vehicle's computer. The service provides a printout or email of the diagnostic codes, the potential causes, and suggested parts and repairs.

How it works: An AutoZone employee will plug a code reader into your car's OBD-II port to get the code. The Fix Finder system then analyzes the code against a database of millions of professional repair records to provide a report.
What you get: You will receive a report that lists the trouble codes, along with a list of possible causes and parts that may be needed for the repair.
Limitations: AutoZone employees do not perform a full diagnostic check. They can read the codes, but they cannot perform a professional, hands-on diagnosis of the issue itself.
To get the service: Visit your nearest AutoZone store during business hours with your vehicle.

Why is my check engine light on if nothing seems wrong?

Your check engine light is on even if nothing seems wrong because of a problem with your car's emissions, engine, or transmission systems, such as a loose gas cap, faulty oxygen sensor, or a minor leak in the evaporative system. These issues can be subtle and may not cause obvious drivability symptoms, but they still need to be checked to prevent future damage. The simplest fix is to ensure your gas cap is on tightly, but other causes like a bad sensor or a minor vacuum leak might require professional diagnosis.

Common reasons the light comes on without obvious symptoms

Loose or defective gas cap: A loose or broken gas cap can disrupt the evaporative emissions (EVAP) system, causing the light to come on even if the car drives normally.
Faulty oxygen sensor: The oxygen sensor monitors your engine's air-fuel ratio. A malfunctioning sensor can send incorrect data to the car's computer, triggering the light, but the car may still run fine.
Mass airflow (MAF) sensor issues: A problem with the MAF sensor can lead to a similar situation as a bad oxygen sensor, as it also measures the air entering the engine.
Evaporative Emission Control (EVAP) system leak: Beyond the gas cap, other parts of the EVAP system can leak, causing the light to turn on to indicate an issue with capturing fuel vapors, even if your car seems to be running fine.
Minor engine or transmission problems: A bad spark plug or a slight issue with a sensor can trigger the light without causing a significant change in performance that you would immediately notice.
Defective catalytic converter: While often associated with performance issues, a failing catalytic converter can also cause the light to come on.
Software glitch or system error: In rare cases, a bug in the car's computer (ECU) can cause the light to come on without a corresponding mechanical fault.

What to do next

Check the gas cap: The first and easiest step is to make sure your gas cap is on tightly. If it's loose, tighten it, and the light may turn off after a few driving cycles.
Get the codes read: The best way to diagnose the problem is to have the error code read from the car's computer. You can visit an auto parts store or a mechanic, which is often a free service at auto parts stores.
Consult a mechanic: Even if the car seems fine, the check engine light indicates a potential problem. It's best to have a professional diagnose the issue to prevent any minor problems from becoming major ones.

What is the first thing to check when the check engine light comes on?

The first thing to check when your check engine light comes on is your gas cap. Make sure it's tightened securely, as a loose cap is a common cause for the light to turn on. If the light remains on after you've tightened the cap, check your dashboard for any other warning lights like low oil pressure or overheating and pull over safely if you see any.
Step 1: Check the gas cap

Unscrew the gas cap and then re-tighten it until it clicks.
A loose or damaged gas cap can cause a vapor leak, which triggers the light.
After tightening, the light may not turn off immediately. You may need to drive for a while for the system to reset.

Step 2: Check for other signs of trouble

Look at your dashboard to see if any other warning lights have come on, such as a light for low oil pressure or a temperature gauge that is in the red zone.
Listen for any unusual sounds coming from the engine or notice if there are any changes in how the car is driving, like a loss of power.
If you see other warning signs or notice performance issues, pull over as soon as it is safe to do so and turn off the engine.

Step 3: Get the diagnostic code read

If the light is still on and there are no immediate signs of a serious issue, your next step is to get the car's trouble code read.
Most auto parts stores offer free diagnostic services where they can plug a code reader into your car's port and tell you what the issue is.
This code can provide clues to more complex problems like issues with an oxygen sensor or catalytic converter.

What is the most common cause of the check engine light?

The most common reasons for a check engine light are a loose or faulty gas cap, a failing oxygen sensor, or bad spark plugs. Other frequent causes include a malfunctioning catalytic converter, a dirty mass airflow sensor, or vacuum leaks.
Common causes
This video explains the top reasons for the check engine light: 47sThe Auto BrainYouTube · May 5, 2025

Loose or damaged gas cap: A loose or cracked gas cap can allow fuel vapors to escape, triggering the light. This is often one of the easiest fixes.
Failing oxygen sensor: The oxygen sensor measures the amount of oxygen in your exhaust. If it's failing, it can lead to a drop in fuel efficiency and turn on the light.
Bad spark plugs or ignition coils: Worn-out spark plugs or a failing ignition coil can cause engine misfires, leading to poor performance and triggering the light.
Faulty catalytic converter: This component converts harmful exhaust gases into less harmful ones. A failing catalytic converter can cause a significant drop in fuel efficiency and performance.
Dirty or faulty mass airflow sensor (MAF): The MAF sensor measures the amount of air entering the engine. Contamination or a malfunction can cause the light to come on.
Vacuum leaks: Cracks or leaks in vacuum hoses can disrupt the air-fuel mixture and trigger the light.

What to do

First, check that your gas cap is on tightly and is not damaged.
If the light remains on, you can use an OBD-II scanner to read the specific code from your car's computer to diagnose the problem more accurately.
Some issues, like a loose gas cap or minor vacuum leaks, can be fixed yourself. Others, like a failing catalytic converter or oxygen sensor, may require a professional mechanic.

Kevin Bennett

Company Owner

Kevin Bennet is the founder and owner of Kevin's Autos, a leading automotive service provider in Australia. With a deep commitment to customer satisfaction and years of industry expertise, Kevin uses his blog to answer the most common questions posed by his customers. From maintenance tips to troubleshooting advice, Kevin's articles are designed to empower drivers with the knowledge they need to keep their vehicles running smoothly and safely.