Top 4 Reasons Why Your Check Engine Light Might Turn ON

Top 4 Reasons Why Your Check Engine Light Might Turn ON

Updated on June 23 2024

It could happen to the best of us. Whether you have a new or used car, minor system faults will cause the check engine light or the Malfunction Indicator Lamp (MIL) to, out of nowhere, suddenly illuminate or turn ON.

And when this happens, the best thing to do is NOT to panic. On the contrary, you should be thankful for the check engine light and your vehicle’s OBD-II or onboard diagnostics. Without it, drivers, mechanics, and even ordinary car owners will have no clue of the impending fault within the system, making possible diagnosis and repair next to impossible.

What is OBD-II or OBD-2?

Without sounding like a Harvard engineering professor, OBD-II or OBD-2 is a self-diagnosing feature of the vehicle’s electrical and mechanical components. It also has reporting capabilities which you can access by plugging an OBD-II scanner into the OBD port. Any fault within the system will present itself via the check engine light we know today.

OBD or onboard diagnostics is a standardized protocol for all vehicles (regardless of make and model) manufactured from 1996 onwards. Before OBD came to fruition, car manufacturers used proprietary diagnostic systems, specialized scanning tools or electronic interfaces, and custom error codes to report problems and issues.

When did OBD-II start?

In the 1960s, several international organizations laid the foundation for a standardized diagnosis system for all light and medium-duty vehicles. In 1968, German automaker Volkswagen became the first to use an OBD computer system with scanning capabilities.

By 1979, the Society of Automotive Engineers (SAE), the International Organization for Standardization (ISO), the U.S. Environmental Protection Agency (EPA), and the California Air Resources Board (CARB) recommended a standard set of diagnostic error codes and connectors for all vehicles.

In 1994, California mandated all vehicles sold to come with OBD-II and a series of diagnostic trouble codes (DTC) by 1996. That year, OBD became mandatory for all cars manufactured and sold in the United States. And by 2018, all vehicles are implementing OBD-II via a Controller Area Network (CAN-BUS).

What is the check engine light?

The check engine light is a reminder of specific faults or errors in the vehicle’s many components. It will illuminate its amber, yellow, or orange warning light in the instrument console to warn the driver that something is wrong with the vehicle.

Top 4 Reasons for the Check Engine Light to Turn ON

Since the vehicle CPU governs the OBD-II system, the check engine light can turn ON for literally hundreds upon hundreds of possible faults. Here are the 4 of the most fundamental reasons that may trigger the Malfunction Indicator Lamp (MIL).

1. A loose or broken fuel cap.

Surprise, surprise! The fuel tank and the fuel cap are significant parts of your vehicle’s evaporative emissions system or EVAP. A loose, broken, leaking, or faulty fuel cap causes a low-pressure scenario inside the tank. If the system detects this low-pressure condition, it will trigger the check engine light.

If you suspect the fuel cap is to blame for the check engine light, perform the following steps:

Step 1: Park the vehicle and engage the parking brake.

Step 2: Open the fuel door and turn the fuel cap counterclockwise to remove it.

Step 3: Check the fuel cap for cracks, leaks, or imperfections on the rubberized seals. If you find anything wrong with the fuel cap, replace it immediately.

Step 4: If the fuel cap is in good condition, re-install it in the fuel filler hole by turning it clockwise. You should hear a series of clicking sounds to indicate the fuel cap is tight.

Step 5: Start the engine and drive the vehicle for 10 to 15 minutes to see if the check engine light will turn OFF.

2. A dirty or faulty mass airflow sensor (MAF).

The mass airflow sensor (MAF) monitors the amount of air entering your car’s engine. Data from the MAF sensor goes directly to the engine CPU or ECU (Engine Control Unit). Then, the CPU adjusts the amount of fuel inside the combustion chamber to match the airflow.

If the MAF sensor is dirty, faulty, or clogged with dirt, it will cause symptoms like hard starting (or a no-start condition), tepid acceleration, or engine stalling. Worst, it can also damage the spark plugs, oxygen (O2) sensors, and catalytic converter.

If you think your vehicle has a dirty or faulty MAF sensor, here’s what to do.

  • Park the car and open the hood.
  • Unhook or remove the airbox and the air filter to access the MAF sensor.
  • Grab a can of compressed air to rid the MAF sensor of excessive dust and dirt. DO NOT touch the MAF sensor with your hands or fingers unless you’re in the mood to buy a new one.
  • Re-install the air filter and the airbox, start the vehicle, and determine if the check engine light is still ON.

3. Faulty O2 or oxygen sensors.

Although faulty or worn-out O2 sensors are more prevalent in older cars, even new cars can encounter oxygen sensor problems. Your vehicle can have up to four oxygen sensors depending on the vehicle type and engine size, but most vehicles with four-cylinder engines have two O2 sensors.

The oxygen sensors detect if the fuel/air mixture is lean or rich. It tells the CPU to adjust the air/fuel ratio to produce a cleaner burn. If the air/fuel ratio is off-balance, it can cause your vehicle to spew more toxic substances in the tailpipe and cause the check engine light to turn ON.

Left unchecked, it may damage the engine. It can also cause your motor to burn more gas, which is not good news considering the rising cost of diesel and gasoline fuel.

Your vehicle may run fine when dealing with a faulty O2 sensor. But if your car is running poorly, bring it to the mechanic immediately to address the fault.

4. Worn-out catalytic converter.

Yes, a broken or worn-out catalytic converter may trigger the malfunction indicator lamp. The catalytic converter, umm, converts toxic gasses into less harmful emissions like water, nitrogen, and carbon dioxide.

Age plays a factor in determining the health of your vehicle’s catalysts. Still, specific problems like a leaking head gasket or faulty O2 sensors can prematurely wear out the catalytic converter, too.

Unfortunately, replacing the catalytic converter requires special tools and advanced knowledge, not to mention hefty repair costs. When in doubt, bring your vehicle to a repair shop.


The best way to determine the cause of the check engine light is to use a scan tool. The scan tool will ‘read’ the error code to pinpoint the root of the problem. If you don’t want anything to do with scan tools and OBD-II error codes, your trusted mechanic will shed some light.