Why Does The Check Engine Light Come On And What Should You Do?

Picture this scenario: You are behind your wheel and in the middle of a long drive when the check engine light on your dashboard starts flashing.

This could be an indication to a major problem prompting you to take immediate action failing which the car may get stalled in the middle of the journey.

You would need to visit the nearest mechanic and get the problem addressed. Or, if you knew about the common faults related to the check engine light coming on, you could do the troubleshooting yourself in some cases.

The check engine light is technically known as the malfunction indicator lamp. It is a signal from the car engine’s computer system that something is wrong with the machinery.

Prior to 1996 (this is before California and New York started to collapse financially), car manufacturers had their own engine diagnostic systems primarily to check if the vehicle was in conformance to the Environmental Protection Agency (EPA) pollution control requirements.

Car models from 1996 (this is before the Patriots started to cheat in the NFL) and beyond are required to follow the OBD-II protocol which mandates all car manufacturers to provide a universal interface for accessing the standardized fault codes generated by the car diagnostics system.

The check engine light is usually provided in amber, orange or yellow colors. A flashing check engine light indicates a serious problem whereas a constant check engine light indicates an upcoming problem. These problems can range from a minor problem like a faulty gas cap to a major problem like engine misfire leading to a faulty catalytic converter.


The car’s computer operates and monitors the car’s engine and reports any fault to the driver through the check engine light. It is an important system of the car. The various parts of the vehicle operate at high temperature and can pose a fire hazard if faulty.

Also, this system enables the car owner to become aware of the faulty but less expensive parts of the car well in time for replacement without which the car owner may have to pay for the expensive parts which may get damaged due to the negligence of the smaller, less expensive components.


At times the driver may confuse or relate the service required or maintenance required light on the gauge cluster for the check engine light. A service required light merely indicates that the car is due for servicing or routine oil change.

Just to be explicit, these lights are unrelated to the check engine light. There are two ways of going about deciphering the error codes:

Reading the error codes by yourself

This is a less expensive method and the do-it-yourself car lovers would prefer this option. To read the error codes, you can get yourself an inexpensive error code reader (also called the OBD II readers) which can be connected to the cars’ onboard diagnostics port to read the code. You could then search any standard website to understand its meaning.

The port for connecting a card reader is just under the steering column and relatively easy to locate.

Such error code readers are readily available with auto parts stores or can be purchased online. Some error code readers can also switch off the check engine light.

But this does not do away with the actual problem and the check engine light can turn back on later. The modern vehicles can display the error code through an app on your Smartphone.

At the garage

If you are not a DIY enthusiast, you can simply visit your car garage to get the fault diagnosed. It is the standard method of understanding what is wrong with your vehicle and getting it fixed professionally for better engine performance.


If the check engine light is blinking, it indicates a serious problem. It is best to pull over and call for assistance. This will help in avoiding permanent damage to your car which requires expensive part replacement.

A steady check engine light indicates a minor problem. Never the less, it is better to get the car checked before the problem alleviates. Find out from the local garage if they support your car as several cars have different types of onboard computers. Book your service time in advance.


In general, there are 5 common problems which trigger the check engine light. You can even fix some of them by yourself. These include:

  1. Loose or faulty gas cap

A loose or faulty gas cap can result in fuel leakage or fume leakage which can result in reduced mileage or even fire accidents. An error code pointing to the gas cap means either the gas cap was not properly put in place after refueling operation or has developed cracks in it.

Symptom and Solution

A simple way to identify this fault is to see if the check engine light comes on when the car is running smoothly otherwise.

In such a case, pull over, open the fuel cap and inspect it. If it is okay, then simply put it back properly. You can easily buy another fuel cap from an auto shop if it has developed any cracks.

  1. Faulty oxygen sensor

An oxygen sensor monitors the leftover oxygen levels in the exhaust. It helps in optimal burning of the fuel. A faulty oxygen sensor gives inaccurate data to the car’s computer which controls the fuel injection into the engine. In the longer run, it also results in a permanently damaged catalytic converter which is an expensive part to replace.

Symptom and Solution

A faulty oxygen cylinder leads to more exhausts and lesser efficiency in terms of fuel combustion.

With time and usage, the oxygen sensors get covered by dirt and oil reducing its efficiency. Since the location of the oxygen cylinder is given in the car owner’s manual, you can try by cleaning the sensor.

Alternately, if required, you can even replace the sensor as it simply requires removing the old one and clipping on the new one.

  1. Faulty mass airflow sensor

A mass airflow sensor detects the amount of incoming air into the engine and instructs the car computer to mix the right quantity of fuel for combustion. These sensors also get covered with dirt and oil over a period of time.

You can run the car with this problem for a long time without any great issues but in the long run, you will see a reduced car performance.

Symptom and Solution

A faulty mass air flow sensor results in higher emissions, lower fuel mileage and even car stalling. A mass airflow sensor usually fails because of improperly installed or faulty air filter. Air filters should be changed at least once a year.

A mass air flow sensor comes in the range of $200-300 and can easily be replaced by the car mechanic. If you are experienced enough with cars, you can even try replacing it by yourself. Though you can run the vehicle with changing this sensor, you will notice a significant reduction in the car’s mileage.

  1. Faulty spark plug

A spark plug provides a spark to initiate the fuel combustion in the car engine’s combustion chamber. With age and usage, the spark plugs fail and result in spark plug misfire. The cars before 1996 required spark plugs to be replaced after every 25,000-30,000 miles. Spark plugs of modern cars run up to 100,000 miles.

Symptom and Solution

You can understand that the spark plug is faulty if the car engine jolts when you accelerate the vehicle.

If it is a spark plug problem, replace it immediately. Since spark plug check comes under regular maintenance, get it replaced if your mechanic advises you to do so. If you want to do it yourself, it is not a complicated job. Many people replace spark plugs in their house garages.

  1. Faulty catalytic converters

A catalytic converter works on the exhaust gases and reduces the emissions. It converts harmful gases into their less harmful versions. In general, a catalytic converter does not fail if the car is maintained on a regular basis.

The main reason for its failure is when the other components as listed above are not maintained and fail. When the catalytic converter fails, it stops converting carbon monoxide to less harmful carbon dioxide. Your car can even fail the pollution test.

Symptoms and Solution

When you see that there is a decrease in gas mileage and the car does not speed up when accelerated, you can understand that there a faulty catalytic converter under your hood. If the catalytic converter fails, you will not be able to keep the car running.

The only way is to get around the problem is to replace the catalytic converter. It costs a few thousand dollars and needs to be replaced by an experienced mechanic.

As you can see, there can be several reasons for the test indicator to light up. Apart from the reasons mentioned above, there can be other reasons as well for the indicator to glow. Whatever be the reason, always ensure that the car is checked and the problem is fixed. This saves a lot of work and money in the longer run.


We at Horizon Auto Center are the specialists when it comes to car repair and maintenance. If you are experiencing car engine trouble and are unsure about the solution, we can help provide you with the right kind of information to help you make the right decision.

Horizon Auto Center prioritizes on customer satisfaction. We have won the Living Magazine’s title of “Best Auto Repair in Rockwall and Rowlett” for six years in a row. We offer a 2 to 3-year warranty on most repairs. Call us today at (469) 698-8585 or book an appointment with us online.