10 Reasons The Check Engine Light Comes On (& the malfunction indicator)

You’re on the road for the vacation that you’ve been longing for. You’re already near your hotel when the check engine light comes on, flashing 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 turned out to be a minor issue that took 5 minutes to fix. So, you ignored it once more, thinking of the warm, inviting bed waiting for you at the hotel instead.


You hear 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. So, the next time it turns on or flashes on your dashboard, take (the right) action.

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 or any closely related issue. Most of the time, this light is triggered only by a minor issue; however, serious malfunctions can also cause the symbol to flash on as well (which, if ignored, can cause fatalities).

Depending 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 of the car’s computer that automates processes and 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 CRV, Jeep, 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. These codes can be read by electronic scan tools (e.g., code readers or diagnostic computers). Today, 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.

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

If the trouble code received is classified as severe, the light symbol could either blink in pulses or light up as a different colour, depending on the car make and model. Honda CRV, Hyundai Elantra, Ford, Chevy, Jeep, Toyota Camry, and VW are some examples of vehicles whose light flashes when the malfunction is severe (e.g., 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 one or more malfunctions in the engine that should be addressed. 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, ranging from minor to critical ones, it can get quite confusing.

Your best bet at troubleshooting on your own would be to buy a code reader that you can link to your car’s onboard diagnostic port or utilize a modern car’s system that 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, which is why it’s important to consult a mechanic even if the trouble code reflects a minor issue.

What is the most common reason for the light coming on? 

To help you narrow down your search, we’ve listed down the most common reasons that trigger the light. This information was gleaned from the 2017 Vehicle Health Index report from CarMD, 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 air-fuel balance. Your car combines gas and oxygen in the fuel as it runs, and 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, which will affect the car’s performance negatively.

When it runs lean, there is too much air and not enough fuel. It leads to sluggish acceleration, decreased power, and faulty ignition because there is not enough power created upon fuel combustion. On the other hand, when it runs rich, there is too much fuel and not enough air. It causes improper fuel combustion (due to lack of air) and will make your engine use up fuel quickly. Some signs of it running rich are a strong fuel smell from exhaust, inaccurate gas mileage (frequent gas refills), dark or black exhaust, increased toxic emissions, and poor car acceleration and performance.

Replacing your car’s oxygen sensor should not be delayed because a faulty one can also cause damage to other parts, such as the MAF sensor, fuel injectors, ignition coils, spark plugs, etc.

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, 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) to convert harmful emissions into less toxic forms.

The root cause of this issue is clogging. Usually, converters don’t get clogged until after a decade, when it’s already corroding or rusting with age, but early clogging can also happen if there’s unburned fuel or coolant and oil leak that gets stuck in the honeycomb structure.

A faulty or clogged catalytic converter will restrict the proper flow of the exhaust, leading to reduced performance and increased fuel consumption. Signs of clogging include excessive heat under the vehicle, distinct sulfuric odour from the exhaust, sputtering, exhaust leaks, and reduced acceleration. In a year, 6.75% of repairs (360,827 cases) are due to this issue, and it ramped up the most expensive annual total cost of $429.4 million—catalytic converters are durable and contain precious metals, which is why they’re pricey, with an average cost of $1,190 (people steal these from cars).

To fix this, some can make do with cleaning the converter to get rid of clogs. However, if there is extensive clogging and corrosion, the best choice is a replacement. It’s recommended by automakers and mechanics to replace your old converter with an OEM catalytic converter, which is of higher quality and has more catalyst metals inside. Some models also use a particulate filter that has 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 the replacement of ignition coils and spark plugs. These are parts of the car’s ignition system, which ignites the fuel and starts the car. The ignition coil is an induction coil that amplifies the voltage of the car’s battery into thousands, which allows the coil to create a spark in the spark plugs. Then, the spark plug, as the name implies, 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.

Between the two, the spark plug is the one that’s more likely to be damaged. Ignition coils are durable, capable of lasting 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. Through time, there is a change in the distance between the electrodes in the spark plug, which 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/or 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. It’s obvious that all of these, if left unchecked, can get dangerous. If the light came on while driving and you noticed any of these signs, get it checked immediately and replace the ignition coil and spark plugs if needed.

4. Loose fuel cap

Fuel cap keeps dirt, dust, and other debris from contaminating your gas and prevents any fuel or vapour leakage. Since this is frequently opened whenever you fill up your tank, the cap itself may loosen through time, which may 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 cost on fuel, issues on evaporative emissions or vapour leaks, disturb the fuel-air balance, and worst, it can lead to a fire.

Believe it or not, there were 222,376 cases of loose fuel caps in a year. So, to be safe, 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 every time the engine light comes on.

5. A need to replace mass air flow (MAF) sensor

The Mass Air Flow (MAF) sensor measures the amount of airflow that’s going into the engine. This measurement is vital because the engine relies on it to calculate minute adjustments and regulate other processes, such as achieving a proper fuel-air balance. The car’s computer also uses this measurement for other vital calculations and minute changes so the engine can run smoothly.

With an inaccurate MAF sensor, the car’s system would get the wrong mass airflow measurement, which will lead to it running lean (i.e., not enough fuel and too much air) or running rich (i.e., 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 vapours that must not readily escape into the atmosphere or to the other parts. The Evaporative Emission Control (EVAP) system is part of the car’s automotive gasoline engine, which is vital for the control of these vapours.

It houses, absorbs (via charcoal canister), and periodically purges accumulated vapours from the fuel system for a safe and controlled purging. When a certain threshold of vapour is reached inside the tank, the EVAP control valve opens up to release it into the charcoal canister, where the emissions are absorbed. Furthermore, the purge valve opens to release vapours 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, 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 some need to be kept cool. For engines, a certain temperature must be achieved, and a certain threshold must not be exceeded. Most modern ones must be at around 200℉ for it 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, preventing the flow of the coolant to the radiator. This leads to an increase in temperature. And when it is hot enough, the valve opens up so the coolant can travel through the radiator to prevent a further increase in temperature.

With a faulty thermostat, your car will lose power and mileage due to a cold engine, or it can suffer permanent damage due to overheating. When it is too cold, fuel is wasted because the temperature is not ideal for fuel combustion, which leads to the system prompting more fuel usage (thus, wasting it). And when an it overheats, it may cause irreversible damage (such as cracking of parts, broken exhaust valves and coolant leak, among many others).

8. A need to replace fuel injectors 

Fuel injectors, as the name implies, inject (i.e., sprays) 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, which ensures the mixing of fuel with air before its introduction 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, 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 car, 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 compared to petrol ones. The Exhaust Gas Recirculation Valve (EGR) was introduced in the 1990s to automobiles to reduce the amount of nitrogen oxide in the exhaust gas.

What EGR does is it passes the gas back into the intake manifold of the engine (i.e., recirculation), where it is combusted again, resulting in reduced gas emissions. This cycle allows a significant reduction in emissions, which is vital to reach the standards set for safety. One that cannot recirculate emissions will be too harmful to human health and the environment.

Over time, carbon deposits can clog the EGR valve, which 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 camshaft position sensor (CMP)

A camshaft position sensor (CMP) counts the rotations of the camshaft when the engine runs. The CMP works together with the car’s powertrain control module (PCM) to determine the position of the camshaft in relation 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.

Inaccurate readings due to a faulty CMP sensor will cause the PCM to misguide the fuel injector, thereby affecting 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).

Can low oil cause it to come on?

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

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

car oil being checked

Is it safe to drive when the light is on?

Technically, you can 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 the code for a loose gas cap. 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 just to be sure.

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

What to do when it comes on

The best thing to do when you’re driving and the check engine light comes on is to 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 (i.e., a loose gas cap). If you can fix it by yourself, you can do so. If the light doesn’t come on again, you’re off the hook!

If, however, you do not have a code reader, or the trouble code given to you was something that you could not fix by yourself, it will be best to just pull over, kill the engine, call the nearest repair shop, stay calm, and do not drive while the light is still on.

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