10 Reasons The Check Engine Light Comes On

You’re on the road to the vacation you’ve been longing for. You’re already near your hotel when the check engine light flashes on your dashboard. What do you do? Tapping on the steering wheel, you contemplate if you should get it checked.

The nearest repair shop is at least an hour away. Your hotel is nearer, only 15 minutes away. You’re bone-deep tired.

The first time you got fussy over the symbol, it became a minor issue that took 5 minutes to fix. You ignored it once more, thinking of the warm bed waiting for you at the hotel instead.


The most common reason the check engine light comes on are issues with the emissions/exhaust system, which will indicate a malfunction. Other common reasons for the check engine light coming on include oxygen sensor issues and a loose fuel cap.

We also cover the following topics:

  • What the light means
  • Other common reasons
  • How can you troubleshoot?

Table of Contents

Reasons the check engine light comes on? 

To help you narrow down your search, we’ve listed the most common reasons the engine light comes on. This information was gleaned from the 2017 Vehicle Health Index report from CarMD.

It’s a corporation of professional technicians and engineers that offer solutions for the automotive industry. In total, 5.3 million repairs were recorded. The list below accounts for 62% of check engine light repairs.

1. A need to replace the oxygen sensor

The most common issue is oxygen sensor related, accounting for 8% of all the repairs with 427,647 cases. The average repair cost was $259, and the total cost was $110.6 million!

An oxygen sensor is necessary for keeping the engine’s air-fuel balance. Your car combines gas and oxygen in the fuel as it runs. The sensors are the ones responsible for maintaining a proper ratio.

This directly influences your car’s performance, ignition, and emission control. A faulty sensor will disrupt this balance, causing your car to run lean or rich. It will affect the car’s performance negatively.

There is too much air and not enough fuel when it runs lean. It leads to sluggish acceleration and decreased power. It can also cause faulty ignition because there is not enough power created upon fuel combustion.

On the other hand, there is too much fuel and not enough air when it runs rich. It causes improper fuel combustion due to a lack of air. It will make your engine use up fuel quickly. Some signs of it running rich are a strong fuel smell from the exhaust or inaccurate gas mileage.

Be ready to experience frequent gas refills. Dark or black exhaust and increased toxic emissions are other signs. Are you experiencing poor car acceleration and performance? Make sure your oxygen sensor is doing alright.

Replacing your car’s oxygen sensor should not be delayed because a faulty one can also cause damage to other parts. Vulnerable parts include the MAF sensor and the fuel injectors. Check out the ignition coils and spark plugs as well.

2. A problem with the catalytic converter

Vehicles release toxic gas emissions as a byproduct of processing diesel or gasoline. For this, a vehicle relies on its catalytic converter. It is a vital part of the exhaust system.

It has a ceramic honeycomb structure coated with metals (e.g., platinum, rhodium, and palladium). As the name implies, these metals act as catalysts and allow oxygen to react with toxic fumes (e.g., CO and NOx). It converts harmful emissions into less toxic forms.

The root cause of this issue is clogging. Converters usually don’t get clogged until after a decade, when it’s already corroding or rusting with age.

Early clogging can also happen if there’s unburned fuel or coolant and oil leak that gets stuck in the honeycomb structure.

car dashboard

A faulty or clogged catalytic converter will restrict the proper flow of the exhaust. It leads to reduced performance and increased fuel consumption. Signs of clogging include excessive heat under the vehicle and a distinct sulfuric odor from the exhaust.

Sputtering and exhaust leaks are typical signs too. 6.75% of repairs (360,827 cases) are due to this issue in a year. It ramped up the most expensive annual total cost of $429.4 million.

Catalytic converters are durable and contain precious metals. It is why they’re pricey, with an average cost of $1,190. People steal these from cars.

Some can clean the converter to get rid of clogs to fix this. If extensive clogging and corrosion are extensive, the best choice is a replacement.

Automakers and mechanics recommend replacing your old converter with an OEM catalytic converter. It has higher quality and more catalyst metals inside.

Some models also use a particulate filter with a 99% increase in efficiency of catching ultra-fine particles.

under the hood of a car

3. Trouble with the ignition coil & spark plug

In total, 333,030 repairs (6.23%) were for replacing ignition coils and spark plugs. These are parts of the car’s ignition system, igniting the fuel and starting the car. The ignition coil is an induction coil that amplifies the voltage of the car’s battery into thousands.

It allows the coil to create a spark in the spark plugs. Then the spark plug ignites the air-fuel mixture in the engine through the electricity (i.e., spark) formed with the help of the ignition coil. This process starts up your car’s engine.

Of the two, the spark plug is the one that’s more likely to be damaged. Ignition coils are durable and can last after 100,000 miles of mileage or more. But once the damage is done to the spark plugs, the ignition coil follows because the two are part of one system.

Over time there is a change in the distance between the electrodes in the spark plug. It causes a decrease in voltage or electricity formed upon ignition. This ultimately leads to difficulty in starting up your car.

Here are the signs of faulty ignition coils and spark plugs:

  • misfiring, reduced car power (i.e., acceleration),
  • increased fuel use,
  • difficulty in starting the car,
  • rattling noises during idle,
  • loud engine,
  • backfiring exhaust,
  • increased emissions,
  • and leaks

If left unchecked, it can get dangerous. If the light came on while driving and you notice any signs, get it checked immediately. Replace the ignition coil and spark plugs if needed.

Reasons for spark plugs going badNote
CombustionOil can leak into the combustion chamber & it can cause the spark plug to fail.
OverheatingRepeated overheating can cause it to fail.
Improper spark plug gapIt needs to be calibrated perfectly for the best performance.
car dashboard in black car

4. Loose fuel cap

A fuel cap keeps dirt and dust from contaminating your gas and prevents any fuel or vapor leakage. Since this is frequently opened whenever you fill up your tank, the cap itself may loosen through time. It may even come loose as you drive.

But human error may also come into play. People sometimes forget to tighten the cap when they’re in a hurry.

A loose (or missing) cap is a small but dangerous precursor of tragedy. It will cause extra fuel costs, issues with evaporative emissions, or vapor leaks. It can also disturb the fuel-air balance and lead to a fire.

Believe it or not, there were 222,376 cases of loose fuel caps in a year. If your car’s gas cap keeps on loosening a couple of times a month, it’s best to replace it rather than tighten it.

While it may seem fine to tighten it every time the engine light comes on, it’s safer to have it replaced.

5. A need to replace the mass airflow (MAF) sensor

The Mass Air Flow (MAF) sensor measures the amount of airflow going into the engine. This measurement is vital because the engine relies on it to calculate minute adjustments.

It also regulates other processes, such as achieving a proper fuel-air balance. The car’s computer uses this measurement for other calculations and minute changes so the engine can run smoothly.

The car’s system would get the wrong mass airflow measurement with a faulty MAF sensor. It will lead to it running lean (not enough fuel and too much air) or running rich (too much fuel and not enough air). The symptoms of a damaged MAF sensor are similar to those of a faulty oxygen sensor:

  • Overall poor car performance,
  • inaccurate gas mileage,
  • and darker or more toxic exhaust.

6. Faulty evaporative emissions (EVAP) purge solenoid or control valve

When fuel sloshes inside the tank, it generates large amounts of vapors that must not readily escape into the atmosphere. The Evaporative Emission Control (EVAP) system is part of the car’s automotive gasoline engine. It is vital for controlling these vapors.

It houses, absorbs (via charcoal canister), and periodically purges accumulated vapors from the fuel. The system is important for safe and controlled purging. When a certain threshold of vapor is reached inside the tank, the EVAP control valve opens up to release it into the charcoal canister.

There, the emissions are absorbed. Furthermore, the purge valve opens to release vapors into the combustion chamber for the ignition process.

Signs of a faulty EVAP purge valve (either it cannot be closed or cannot be opened) include the following:

  • unsuccessful or rough engine idle,
  • damage to fuel lines and hoses,
  • excess emissions, low gas mileage,
  • the damaged catalytic converter,
  • difficulty in starting the car after gas refills,
  • and vacuum leak.

7. Trouble with thermostat

Some machines need to be hot to function well, while others need to be cool. A certain temperature must be achieved for engines, and a certain threshold must not be exceeded. Most modern ones must be at around 200℉ for them to operate optimally.

A thermostat is a temperature-controlled valve that regulates the temperature. When the engine is too cold, the thermostat housing is closed.

It prevents the coolant flow to the radiator. This leads to an increase in temperature. When it is hot enough, the valve opens up so the coolant can travel through the radiator to prevent further temperature increases.

Your car will lose power and mileage due to a cold engine with a faulty thermostat. It can suffer permanent damage due to overheating.

Fuel is wasted when it is too cold because the temperature is not ideal for fuel combustion. It leads to the system prompting more fuel usage (wasting it). And when it overheats, it may cause irreversible damage. Damage includes:

  • cracking of parts,
  • broken exhaust valves,
  • coolant leak,
  • among many others.

8. A need to replace fuel injectors

As the name implies, fuel injects the fuel into the inlet valve or port. These are electronically-controlled valves that are important in achieving the proper air-fuel mixture.

These injectors release the right amount of fuel at a controlled rate. It ensures fuel mixing with air before introducing it to the combustion chamber. This makes the combustion process more efficient.

Injectors become faulty when they’re clogged or damaged. These valves are likely to get damaged over time because they’re located at the engine’s head. In there, they’re constantly exposed to heat and pressure.

When the injector is faulty, it may either hyper-perform or under-perform. The former leads to too much fuel (it will run rich), while the latter leads to too much air (it will run lean).

Some signs of bad fuel injectors are:

  • misfiring,
  • noisy idling cars,
  • leaks, loss in mileage,
  • and stalling.

9. Clogged or damaged exhaust gas recirculation (EGR) valve or port

Nitrogen oxide (NOx) is a toxic gas emitted by diesel engines in higher amounts than petrol. The Exhaust Gas Recirculation Valve (EGR) was introduced in the 1990s to automobiles. Its purpose was to reduce the amount of nitrogen oxide in the exhaust gas.

EGR passes the gas back into the engine’s intake manifold, where it is combusted again, resulting in reduced gas emissions. This cycle allows a significant reduction in emissions, which is vital to reaching safety standards. One that cannot recirculate emissions will be too harmful to human health and the environment.

Over time, carbon deposits can clog the EGR valve. It will lead to further damage. You can either clean or replace the valve before using your car again. One with a faulty EGR valve will not pass the emissions test and must not be driven around. Signs of a broken or clogged EGR valve include the following:

  • dark smoke or emissions,
  • poor mileage,
  • rough idling,
  • reduced power and acceleration,
  • and poor engine health and performance.

10. A need to replace the camshaft position sensor (CMP)

A camshaft position sensor (CMP) counts the rotations of the camshaft when the engine runs. The CMP works with the car’s powertrain control module (PCM) to determine the position of the camshaft relative to the crankshaft. This information is delivered to the car’s computer and used by the PCM to control the fuel injector and the ignition system.

Due to a faulty CMP sensor, inaccurate readings will cause the PCM to misguide the fuel injector. It affects the air-fuel ratio and the ignition system. Signs of a bad CMP sensor are as follows:

  • ignition problems,
  • jerking car movements,
  • rough idling,
  • engine stalling,
  • poor mileage (running rich),
  • and reduced power and acceleration (running lean).
check engine light


You hear a sputter as your car slows down. Smoke is coming out of the hood of your car. It caught on fire. It turns out the gas tank was leaking, and the engine was misfiring.

If only you knew better. You would’ve never ignored the check engine light. The next time it turns on or flashes on your dashboard, take (the right) action.

Here are some engine codes you could receive if you hook it up and get it tested.

  • P0301
  • P0302
  • P0303
  • P0304
  • P0305
  • P0306


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check engine light is on in a car

What does the check engine light mean?

Also known as the malfunction indicator lamp, the check engine light serves as your warning when there’s a malfunction. Most of the time, this light is triggered only by a minor issue. Serious malfunctions can also cause the symbol to flash on. If ignored, it can cause fatalities.

The color of the flashing light depends on the car manufacturer. Its symbol may appear as a yellow, orange, or amber outline of an engine. Or it may be words such as “check,” “check engine.”

It is connected to your car’s onboard diagnostic system, a vital part that senses an issue. It has been the protocol since 1996 for all automobiles to have this system for a standardized and easier detection of issues. It doesn’t matter if you own a Honda or a Volkswagen. All vehicles are equipped with this system.

The computer stores a list of trouble codes in the diagnostic system that corresponds to a specific malfunction in your engine. Electronic scan tools can read these codes (code readers or diagnostic computers).

Systems of some modern cars can be connected to an app, so you can easily access the code via smartphone. However, it still takes a professional mechanic to diagnose and pinpoint the exact cause before repairing the issue.

check engine light on in dashboard

When to know if it’s a minor or severe malfunction?

Suppose the trouble code received is classified as severe. In that case, the light symbol could either blink in pulses or light up as a different color. It depends on the car’s make and model.

Honda CRV and Hyundai Elantra are some examples of vehicles whose light flashes when the malfunction is severe. An example could be misfiring. Volkswagen’s symbol turns red in some cases when the problem is related to low coolant content or leaks.

Why is it on?

As the name “malfunction indicator lamp” suggests, the light is on because the car’s diagnostic system detected a malfunction. One or more malfunctions in the engine should be addressed when you see the light.

The system matches a certain trouble code and lights to warn you. But given the long list of diagnostic trouble codes that can trigger, this light can get quite confusing. Issues range from minor to critical ones.

Your best bet at troubleshooting on your own would be to buy a code reader. You can link it to your car’s onboard diagnostic port or utilize a modern car’s system. It allows owners to view the code on their smartphones.

Once you’ve got your trouble code, you can surf the net and narrow down the reasons behind the problem.

However, underlying issues can mask the true cause. It is why it’s important to consult a mechanic even if the trouble code reflects a minor issue.

car oil being checked


Can low oil cause the check engine light to come on?

A case of having low oil would not trigger the light on by itself. A different symbol on the dashboard serves as a warning for low oil. It is usually an outline of a traditional oil can with oil dripping out its mouth.

However, there is a chance that constant and prolonged low oil pressure can trigger the light. It’s only the case if parts of the engine were damaged, such as a faulty oil pump or fuel injector. If it’s only low oil and there’s no damage, the light will not turn on.

Is it safe to drive when the check engine light is on?

You can technically still drive even with the light turned on, as long as your car is still running smoothly. But is it safe? Let’s consider two situations:
Your car was operating smoothly, and the check engine light came on while driving.
You had an onboard code reader, which gave you a loose gas cap code.

After tightening the cap, the light stayed off throughout the drive. For this situation, it’s safe to drive to your destination but make sure to come around an auto repair shop to be sure.

Your car has been acting weird for the past few weeks. Maybe you experienced dark exhaust or poor mileage. The warning light repeatedly flashed on the dashboard while you were driving. You had no code readers in your car and just assumed it was just a minor issue like before. The light continues to come on and off as you drive. In this situation, it is not safe to drive a car anymore.

What to do when the check engine light comes on?

Maybe you’re driving, and the check engine light comes on. The best thing to do is pull over and have a code reader relay the trouble code to you.

This way, you’ll be able to determine if the issue is something you can fix yourself (a loose gas cap). If you can fix it by yourself, you can do so. You’re off the hook if the light doesn’t come on again!

There are situations where you do not have a code reader. Maybe the trouble code given to you was something you could not fix by yourself. It will be best to pull over. Stop the engine, and call the nearest repair shop. Stay calm. Do not drive while the light is still on.