Check engine light: Meaning, causes & how to reset it

Check engine light: Meaning, causes & how to reset it
If you're a driver, chances that at some point you've been worried by the check engine light on your dashboard are high.
What does this scary light mean, though?
The check engine light illuminates whenever your car's internal computer discovers a problem with the car's engine or transmission. Unfortunately, many modern cars could have a long list of reasons, ranging from a loose gas cap to a faulty catalytic converter.
So, should you pull over right away, or is it okay to keep going?
Let's delve into the meaning behind this dashboard warning.

What is the check engine light?

The check engine light, also known as the Malfunction Indicator Lamp (MIL), is a warning light that indicates engine and transmission system problems. This symbol prompts you to have a diagnostic check to prevent further damage.
This light is often displayed on the dashboard in one of two forms — either a car’s engine icon and the word “check” or just two words “check engine.”
Due to the many possible reasons and the rather uninformative nature of the light itself, it’s one of the most frustrating indicators. Additionally, in some countries, just having the light on will mean that your car cannot pass the required technical inspection for registration or renewal.
Car dashboard with the check engine light on
If it just turned on, it’s unlikely that you’ll run into trouble instantly. But the flashing check engine light is a different story. This may indicate a more serious problem that requires immediate action.
In any case, it should be considered as car warning lights for you to get your car to a repair service as soon as possible.

Common reasons the check engine light illuminates for

There are two main ways you can narrow down the reasons for the check engine light – by using an OBD2 diagnostics tool or by going through the list of the most likely occurrences. While the former is much more reliable and accurate, the latter does work as well. 
Here are some of the most common reasons that may cause the check engine light to come on:
Faulty oxygen (O2 or lambda) sensor
The oxygen sensor measures the emissions from the fuel mixture, which is then transmitted to the engine control unit. If the mixture is improper, the control unit changes the ratio of fuel and air.
A faulty oxygen sensor means that the control unit cannot properly change the fuel mixture, which can cause many issues, such as engine misfires, rough idling, or bad fuel economy. As such, a malfunctioning sensor can be one of the more common reasons for the check engine lights to pop up.
Faulty mass air flow sensor
The Mass Air Flow (MAF) sensor measures the amount of air entering the engine. It helps calculate how much fuel is needed for efficient combustion. This ensures the car runs smoothly by maintaining the right air-to-fuel ratio.
A faulty MAF sensor can disrupt this balance, leading to an improper mixture of air and fuel. This may cause issues such as erratic engine performance, decreased fuel efficiency, increased emissions, or even put your car into limp mode. Consequently, a malfunctioning MAF sensor is a common reason for activating the check engine light.
Mass air flow sensor
Ignition coil failure
Ignition coils send the voltage to spark plugs. A malfunctioning ignition coil may not send enough voltage to the spark plugs, which may cause an engine misfire, car shakes when idling, and other changes to how your vehicle runs. 
Since ignition coils directly affect the engine, the associated dashboard light will illuminate if one of them is failing. Replacing them should fix the issue and resolve most engine issues.
Spark plug issues
A spark plug takes the voltage from ignition coils and produces the spark that ignites the fuel. If worn out, an adequate spark may not be produced, which may fail to ignite the mixture. 
Almost identically to ignition coils, failing spark plugs will cause rough idling and engine misfires. They’ll also cause the check engine light to illuminate. Additionally, it may also cause fuel to leak through the exhaust pipe due to it not being ignited properly.
Failing fuel injector
As the name suggests, fuel injectors push fuel into the combustion chamber. Other parts, such as spark plugs, take over the process to complete the combustion cycle.
Faulty fuel injectors will either push too little or too much fuel into the combustion chamber, causing the engine to run too lean or too rich. Both cases may affect engine speed and work efficiency and cause misfires or rough idling.
If the check engine light is on and unusual sounds (such as hissing) come from the engine compartment, a faulty fuel injector could be the culprit.
Car engine bay with a mechanic's hand holding fuel rail with fuel injectors
Loose gas cap
One of the easiest issues to fix is a loose gas cap. The fuel pump requires a complete seal in the fuel tank to operate correctly. However, a loose gas cap will cause air to disrupt it. In turn, the mixture may receive more oxygen, which may cause misfires or rough idling.
In general, any malfunctioning oxygen management part (such as the mass air flow sensor) will cause the check engine dashboard warning lights to illuminate.
Luckily, often, all it takes to fix a loose gas cap is to simply tighten it up. If it’s broken, getting a replacement is usually extremely cheap, so you could consider yourself lucky if a gas cap is the reason for a check engine light.
Catalytic converter issues
If a loose gas cap was the easiest issue, catalytic converter issues are one of the worst possible reasons for the engine warning lights to illuminate. They filter and clean emissions for which precious metals are utilized. 
Due to the precious metals used, catalytic converters are one of the most expensive parts to replace. Leave them as the last possible reason for the check engine lights.
Scheme showing what's inside in the car catalytic converter

Why is the check engine light flashing?

A flashing check engine light means a serious problem. If you see a blinking light, you'd better pull over right away before any serious damage has occurred to the car.
The problem could be related to overheating, low oil pressure, misfiring, or other issues that need immediate attention.
A diagnostic test helps to identify the true cause from numerous reasons behind a flashing check engine light to prevent potential irreversible damage to your vehicle. To fix the issue, it’s advisable to make an immediate service appointment with a qualified mechanic.

Understanding check engine light codes

While the true check engine light meaning is usually almost impossible to deduce from the dashboard, there’s an easy fix to it – OBD2 diagnostic scanners.
These devices connect to the car’s OBD2 port to retrieve information from the vehicle’s computer.
If the check engine light is illuminated, an OBD2 scanner can retrieve a diagnostic trouble code. This code will be more descriptive than the check engine light alone, as it will point to an exact issue.
Some OBD2 diagnostic tools can also retrieve manufacturer specific codes, which are even more informative and precise. As such, these tools are lifesavers if you want to diagnose check engine light issues.
Here are some of the most common error codes you’ll find if the check engine light is on:
  • P0130: Oxygen sensor malfunction
  • P0131: Oxygen sensor low voltage
  • P0171: (Fuel) system is too lean
  • P0172: (Fuel) system is too rich
  • P0173: Fuel trim malfunction (means the fuel-to-air ratio is off)
  • P0200 to P0201: Fuel injector circuit malfunction. P0200 indicates a general malfunction, P0201 indicates malfunction in cylinder 1. 
  • P0300 to P0303: Indicates engine misfires. P0300 indicates misfire of multiple cylinders, P0301 to P0303 is related to cylinder 1 to 3. 
  • P0351 or P0352: Fault in ignition coils A (P0351) or B (P0352). Either can also indicate a general circuit malfunction. 
  • P0420: Low catalytic converter efficiency
  • P0430: General catalyst system malfunction
  • P0430: General catalyst system malfunction
There are always more possibilities and codes, but using an OBD2 scanner can help you understand the source of the issue and its severity. In these cases, the scanner removes all the guesswork from the check engine light issue.

How to reset the check engine light?

Resetting the check engine light is a straightforward process if you know the right steps. Let's explore the most popular ways to turn off this warning light:
Continue driving and let it clear itself
This is the easiest method. After addressing the initial issue, the car's computer might need time to clear the error code.
Keep in mind the system updates regularly. Driving normally for three days, or sometimes more, could turn the light off.
If the light stays on, the problem might still be there. You might need to try other methods listed here or take your car to a mechanic.
Switching the ignition ON and OFF
This method mirrors the classic IT trick of rebooting a computer. Cycling the ignition might reset your vehicle's systems.
Simply insert the key and turn it ON for 1-2 seconds, then turn it OFF for the same duration. Do this three or four times.
If the warning light stays on, it might signal unresolved issues. In such cases, further diagnostics or professional help may be needed.
Disconnect and reconnect the battery
This method requires a bit of hands-on work. The ECU (Engine Control Unit) manages various engine parameters and stores diagnostic codes. Resetting it by disconnecting the battery can sometimes make these codes inactive and turn off the check engine light.
Follow these steps:
  1. Disconnect the negative battery terminal. This terminal is black or has a minus (-) sign.
  2. Drain any residual energy by pressing the brake pedal, using the horn, or lights for over 30 seconds
  3. Reconnect the battery and start your car as usual
Using an OBD2 scanner
The most reliable way to reset the check engine light is with an OBD2 scanner. This way, you’ll not only reset the check engine light but also read and clear fault codes. Here’s how to do it:
  1. Make sure you've fixed what's causing the check engine light to come on. If you don't, the light will just come back.
  2. Plug in an OBD2 scanner, like OBDeleven, to your car's OBD2 port and connect it to your phone via Bluetooth
  3. Use your phone to scan for trouble codes with the scanner. If it finds any, clear them. This should turn off the check engine light.
  4. If the light stays on, it means there are still issues. Fix these problems and try clearing the codes again.
Get wireless OBD2 scanner
Remember, while resetting the check engine light might seem like a quick fix, addressing the underlying issues is essential to ensure your vehicle is running reliably.
Smartphone screen displaying an OBDeleven diagnostic app with a list of engine faults, placed on the center console of a car next to the gear shift and control dials.

Preventing check engine light issues

Check engine light issues are almost inevitable if you use any car long enough. But there’s a lot you can do to reduce the likelihood of having the dreaded check engine light pop up.
Many of the issues we’ve listed above are general wear and tear. While driving behavior may have an effect, regular servicing and taking care of the car will go a much longer way.
For example, according to the manufacturer's recommendations, spark plugs should be replaced regularly. Some of them may last longer and continue working as intended. However, the manufacturer’s guidelines should be followed as closely as possible to avoid any potential issues.
Other regular maintenance recommendations, such as oil changes, may help in preserving parts of your car that would otherwise force the check engine light to pop up.
Additionally, if you are required to replace some parts of your car, the aftermarket shouldn’t always be the first option. Getting parts manufactured by the OEM will reduce the likelihood of issues, which could otherwise cause the check engine light to illuminate. Car tuning could also cause check engine light issues.
Finally, having an OBD2 scanner at the ready is always a nice touch. Many modern and more advanced OBD2 scanners will give you access to various data points (and even live data) collected by the on-board system.
It’ll be a lot easier to perform preventative car maintenance tasks and to take care of your car if you have all the information you need within hand’s reach at all times.