10 Most Common Reasons Your Check Engine Light Comes On

While the check engine light (CEL) does not always indicate an engine problem, it usually indicates that your vehicle system is triggering a concern. If your check engine light comes on, think of it as a pain signal that something is not quite right and needs further investigating.

It is good to note that a check engine light may not always cause your car to feel or run noticeably different, and alternatively, some issues can cause your car to not run right and not throw a check engine light.

You may be surprised to discover that some of the most common check engine lights that we share below are often very cheap or even free to correct!

How Check Engine Lights Work

Since the late 1970’s, most vehicles have been equipped with a computer system that monitors the operation of numerous components, and after 1995, On-Board Diagnostics 2 (OBD-2) became the standard in the US and a requirement for vehicles after 1996. When the computer finds a problem, it attempts to address it on its own. However, it cannot resolve mechanical issues. When the computer cannot resolve the problem, it illuminates the check engine light.

The check engine light is more than simply a warning—it indicates the severity of the problem. A constant light suggests a minor issue that does not need quick care. What about a flashing light? This indicates a critical and potentially catastrophic situation, and you should stop driving immediately to limit engine damage.

Regardless of the severity of the problem, your vehicle’s computer records an error code in its memory. This code can be diagnosed using a code reader or scanner, but most drivers do not have one, making it difficult to discover the issue on your own.

You used to be able to go to an auto parts store, and they would offer a courtesy scan for any check engine lights, but over the years, many places will not offer this due to liability concerns. It is still worth a try, though, as some places may still do it anyway.

If you are interested in a simple scanner to add to your toolbox, an OBD2 scanner like this is a great addition and can be useful for checking engine lights and other testing and diagnostics.

Common Causes of Check Engine Lights

The check engine light is sometimes only a reminder to perform routine maintenance, but it is usually a symptom of a problem that needs to be addressed. When the light illuminates, you will want to scan the code to understand the severity of the issue and whether it is safe to continue to drive the vehicle.

Poorly Secured Fuel Cap

Your gas cap secures fuel inside the tank until it is needed. Fuel can escape or even evaporate if the cap is unfastened, resulting in wasted gas.

If your check engine light illuminates immediately after refueling, you most likely forgot to put the cap back on or did not tighten it sufficiently. However, if your gas cap is secure, it may indicate a fuel system problem, such as a leaking connector.

Issues With the Catalytic Converter

Your catalytic converter is an essential part of your vehicle’s exhaust system, converting hazardous compounds like carbon monoxide into safer ones like carbon dioxide. When your catalytic converter fails, your emissions system cannot operate properly, causing harmful fumes to be discharged and triggering the check engine light.

Catalytic converters are designed to last; therefore, their failure often indicates a more serious problem that requires repairs to the cause. Common causes include bad spark plugs, unburned fuel in the exhaust, and possibly even structural damage to the converter’s ceramic interior.

Faulty Oxygen Sensors

Your car’s engine operates on a precise air-fuel ratio that is controlled by several components. When these components fail, your engine may receive too much or too little oxygen, which can cause long-term harm to various components. Some cars may have multiple oxygen (O₂) sensors, and each one may throw a different code.

One of the most common symptoms can be poor performance and fuel economy, and quite often the most common causes are one of the oxygen sensors, which monitor the amount of oxygen left after the air-fuel mixture ignites. If this sensor identifies a problem with oxygen consumption or burns out over time, the check engine light will illuminate. This problem will further decrease fuel efficiency and could result in more serious engine damage if it is not fixed.

Failing Spark Plugs

The ignition system in your vehicle generates the necessary heat to ignite the fuel in the combustion chamber. Spark plugs generate heat in gasoline engines. Spark plugs can deteriorate over time, resulting in mistimed or defective ignition and a variety of engine-starting troubles.

Diesel engines employ glow plugs, which do not have the same issues. Spark plug difficulties are more common in older engines because of wear and the type of plugs used. Rotary guys, you will have similar types of concerns with more aggressive symptoms.

Bad Spark Plug Wires

Spark plug wires are necessary for carrying voltage from the ignition coil to the spark plugs, which ignite the fuel in the combustion chamber. When these wires deteriorate or fail, they result in a weak or erratic spark, causing misfires and poor engine efficiency.

The check engine light illuminates when your vehicle’s onboard diagnostics system identifies an issue. While this problem is more common in older automobiles owing to wear and tear, it can occur in any vehicle over time.

Mass Airflow Sensor Failure

Your vehicle’s mass airflow (MAF) sensor is critical for ensuring peak engine performance. It measures the amount of air entering the engine and ensures the optimum air-fuel combination. Some cars also use manifold absolute pressure (MAP) sensors as well and can cause similar failures.

When the MAF sensor malfunctions, it delivers inaccurate data to the engine management unit, causing poor engine performance and activating the check engine light. This failure can cause symptoms such as rough idling, stalling, and poor fuel efficiency. 1990’s cars are the most often afflicted, but all cars with MAF’s can have issues. Sometimes, these sensors just need to be thoroughly cleaned.

Issues With Aftermarket Wiring

While aftermarket electronics, such as alarms, stereos, or cameras, provide protection or some other action, they might occasionally interfere with your vehicle’s electronic systems. Improper wiring, installation, or faulty aftermarket components might cause communication issues between the engine control unit (ECU) and numerous sensors.

This can result in incorrect readings or occasional signal loss, prompting the check engine light to illuminate. Unexpected alarm triggers, battery drain, and irregular electrical activity are some classic indications of a faulty aftermarket alarm.

Vacuum Leaks

A vacuum leak is a common cause of the check engine light coming on. These leaks occur when there is a gap, break, or degradation in the vacuum hoses, intake manifold seals, or other components that require a sealed environment.

Extra air can enter the engine (or metered air can leak out), creating an imbalance in the air-fuel mixture, resulting in rough idling, poor fuel economy, or even engine stalling. This problem will be much more common on older vehicles where there are more vacuum-actuated sensors and components, but modern vehicles can still experience this failure.

Exhaust Gas Recirculation (EGR) Valve Failure

The exhaust gas recirculation (EGR) valve helps to reduce emissions by recirculating a portion of your exhaust gases back into the engine cylinders. If the EGR valve malfunctions, it might result in an incorrect combination of air and fuel, increasing emissions, reducing engine performance, and reducing fuel economy by clogging everything in the intake system. A failed EGR valve may result in engine knocking or a noticeable decrease in performance.

To detect EGR valve faults, you can use diagnostic tools such as OBD-2 scanners and digital multimeters to identify the issue. Then you can identify the necessary repairs needed, which could be cleaning the valve itself, replacing any broken components, and installing a new valve if necessary.

Dead Battery

A weak or dead battery can cause numerous electronic components in your car to fail, resulting in the check engine light turning on. Modern vehicles are extensively reliant on electronic systems, and a failing battery might cause the engine control unit (ECU) to indicate problems. This is also one of those issues that can cause many symptoms and issues without triggering a check engine light at all and is often a good first place to test when troubleshooting.

If your headlights are dim, your engine cranks slowly, or your electrical accessories fail, you may have a failing battery. Some auto parts shops can assess the battery’s condition and charging system with a cycling machine. If you do replace the battery, ensure that all connections are secure and corrosion-free so as to keep the charging performance for as long as possible.

Closing

While these are among the most common check engine lights, most of these can be corrected for free or very cheaply by cleaning the failing component thoroughly. Some parts, however, notably the sensors and valves, should just be replaced. Keeping a diligent eye on your car’s built-in OBD system is good practice if you want your car to last and perform at its best.

As we are all car enthusiasts here, keeping up with these kinds of repairs and maintenance is an easy way to allow us to extract all of the power and reliability our cars will allow!