5 Reasons your Check Engine Light Is On

Why is my check engine light on?

check engine light can be costly
image courtesy of Robert

It happens without warning, and it can happen to anyone at any time. When the check engine light comes on, fear and panic are usually the first reactions of the driver, but this should not be the case. In fact, you should be thankful that the check engine light came on because it was specifically designed to warn the driver of an engine malfunction or an impending breakdown. Thank God for OBD-II, or on-board diagnostics 2 (which is of course better than 1 😉 ).

So, what are the top reasons your check engine light might be on? A modern car engine is governed by sophisticated computers and sensors, but here are the most common causes that may trigger the malfunction indicator lamp or MIL on your instrument panel.

1. Loose/Broken/Cracked Fuel Filler Cap

Is this really possible? Of course! A loose or broken fuel filler cap will usually trigger the check engine light in your car. This is especially true for older cars, but it can happen to new cars as well.

This usually happens after filling the tank with gasoline/petrol/(maybe even diesel?). If you don’t close it properly, then fuel vapor might escape, causing a low pressure scenario inside your fuel tank. You need to understand that the fuel tank (including the fuel cap) is part of the evaporative emissions system or EVAP of your vehicle. The EVAP contains sensors for the fuel level and the fuel pressure, including vent and purge solenoid valves.

fuel fill cap
image credits: Wikipedia


If the pressure inside the fuel tank is low, this means that there is a leak in the system, and the check engine light comes on. The fuel filler cap is usually the first culprit. Then again, there might be tiny unicorns jumping around inside the tank, who knows?

What to Do

  • Park the car in a safe and level area.
  • Pop open the fuel filler door and remove the fuel filler cap by turning it counterclockwise.
  • Check the fuel filler cap for cracks or other surface imperfections. If you find anything unusual with the filler cap, then it is time to replace it immediately.
  • If there is nothing wrong with the filler cap, put it back in place and turn clockwise until you hear repeated clicks. This means that the filler cap is sealed tight.
  • Get back in the car and start the engine. Check if the malfunction light is still on.

Tip 1: DO NOT smoke cigarettes when checking the fuel cap, unless you want to be part of the 4th of July celebration.

Tip 2: The check engine light might not turn off immediately. Drive the car for a few miles and check again. If the check engine light persists, it’s time to visit your favorite mechanic.

2. Faulty/Broken Oxygen Sensor or O2 Sensor

This is one of the most common causes that will trigger the check engine light in your car. The Oxygen (O2) sensor is a critical component of the emissions control system. As your car burns gas, the O2 sensor detects either rich or lean mixtures, and tells the engine computer to adjust the combustion to produce the right mixture of air and fuel.

A rich air/fuel ratio is bad because it means there are a lot of unburned gasses in the exhaust system, causing tons of pollution. However, a lean mixture is not so good either, because this produces more nitrogen-oxide pollutants which can seriously damage your engine in the long run. This also means mediocre performance and poor MPG. This also burns a hole in your pocket. Bummer.

Depending on the make and model, a car can have as many as four oxygen sensors. If any one of those fails, expect the check engine light to come on.

What to Do

  • Your car might run fine even if the check engine light is on and the O2 sensor is failing. However, you should stop driving and bring your car to a mechanic to determine and repair the fault.
  • Want to save money on O2 sensor repairs? You can purchase an OBD-II scanner and replace the sensor yourself.

Tip: The O2 sensor is sometimes located in the most unreachable parts of your engine bay. Not a problem if you’re Plastic Man or Mr. Fantastic, but there are times that you need to remove a few components in order to access the O2 sensor. Make sure you know what you’re doing. When in doubt, call your mechanic.

3. Broken/Worn-Out Catalytic Converters

Catalytic converters need to be replaced at least once or twice during the life of the car. A broken or worn-out catalytic converter will trigger the check engine light in your car. The main purpose of the catalytic converter is to convert toxic gasses into something that is less harmful such as carbon dioxide, water and nitrogen.

However, most people don’t know that accelerated wear and tear of the catalyst is caused by other problems such as a bad oxygen sensor, a leaking head gasket or using the wrong type of engine oil. Whatever the case may be, expect the check engine light to come on when the catalyst fails.

What to Do

Replacing the catalytic converter is going to cost some serious dough. Make sure to buy the right catalyst for your car, and have it replaced by a reputable mechanic.

Tip: Did you know that the main purpose of the oxygen sensor is to protect the catalytic converter from unburned fuel and other contaminants? That is why it is important to replace the O2 sensor immediately when it fails.

4. Dirty/Clogged/Faulty Mass Airflow (MAF) Sensor

We’ve heard this story countless times. The MAF sensor is responsible for monitoring the amount of air that enters the engine. The information is relayed to the vehicle CPU, and the engine adjusts the amount of fuel needed to make an efficient and clean burn.


A dirty, clogged or faulty MAF sensor will cause a variety of engine troubles such as hard starting (or inability to start the engine), engine stalling and poor acceleration. Sometimes, there are no symptoms at all. When left untreated, a faulty MAF sensor can cause damage to the spark plugs, the oxygen sensors and the catalytic converters.

Check out this youtube video from Wesley Harrison (awesome guy) discussing MAF sensors. If the video doesn’t load, here is the link: MAF VIDEO

<iframe width=”560″ height=”315″ src=”https://www.youtube.com/embed/aDlHCjgYf8s” frameborder=”0″ allowfullscreen></iframe>

What to Do

Before replacing the MAF sensor, it is a good idea to clean it first and check for dirt and other contaminants inside the chamber.

  • Park the car and pop open the bonnet. It is a good idea to let the engine cool before touching any part of the engine bay.
  • Remove the air box and the air filter to access the MAF sensor.
  • Use a can of compressed air to clean the MAF sensor. Be careful not to touch or break the sensitive wire inside the chamber. If you break it, then you’re an idiot for doing so. Time to pony up the cash for a new one.
  • When in doubt, consult your mechanic.

There you have it, the top reasons your check engine light might be on. If you want to accurately determine the cause of the check engine light in your car, we highly suggest using an OBD-II scanner before making any necessary repairs. This alone will save you time, money and your sanity.

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