Why does the Check Engine light come on?

Updated: January 09, 2019

It's what almost every motorist has to go through once in a while: the Check Engine or Service Engine Soon light. If this warning light stays on, it means that your vehicle's computer has detected a problem that can affect your vehicle's fuel economy and emissions.

Check Engine - Service Engine Soon light
Check Engine or Malfunction indicator light (MIL)

Let's look closer at why the Check Engine light comes on and explore your options on how to deal with it. First, shortly how it works. Your car has a computer (in the photo) that controls the powertrain (engine and transmission). This computer is called Powertrain Control Module (PCM). It works by monitoring signals from various sensors and adjusting the engine and transmission performance for better fuel economy and lower emissions. This computer has a self-testing capability (called ON-Board Diagnostic or OBD-II). When it detects a fault with one of the systems or sensors, it turns on the Check Engine light on your dash. At the same time, it stores the fault code in its memory. There are a few hundred possible codes. To diagnose the problem, your mechanic will have to connect a scan tool to your vehicle and retrieve the code from the computer.

The engine computer or PCM
The engine computer or Powertrain Control Module (PCM)

The code itself doesn't tell exactly what part is defective, it only tells what system doesn't work properly or what parameter is off. Your mechanic will have to do further testing to find the defective part. Once the problem is repaired, your mechanic will reset the Check Engine light. Of course, the repair could be costly unless it's covered by the warranty. Is there any other way reset the Check Engine light? Is it safe to drive with the Check engine light on? What parts could be covered by the warranty? Is it possible to repair the problem DIY? We will try to answer these and other questions in this article.

What needs to be checked first

Gas cap
Check if your gas cap is tight

If you check your owner's manual, it will probably tell you to check if the gas cap is tight. This is because the Check Engine light may come on if your gas cap is not closed properly. Usually, it happens soon after a fill-up at a gas station. If you do find that the gas cap wasn't tight, close it properly and if there are no other problems, the Check Engine light will reset by itself after a day or two of driving. If the gas cap is tight, there is probably some other problem. If the Check Engine light came on after your car has been serviced, take it back to the repair shop and ask them to re-check it. If you are comfortable doing basic checks under the hood, check your oil level (see how), see if the battery terminals are tight, if the air filter box is closed properly or if anything appears to be loose or disconnected under the hood. You can find a map of your engine compartment and instructions on how to check the oil level in the Maintenance section of your car's owner's manual.

Is it safe to drive with the "Check Engine" light on?

It really depends on what the problem is. It could be something minor, like a loose connector or low battery voltage, but it also could be a more serious issue that could cause more damage to your vehicle. In worse cases, a car may stall or lose power. We recommend having your car checked out as soon as possible to be on the safe side. If the Check Engine light is blinking repeatedly, it means that the engine computer has detected that your engine is misfiring, which means that some of the engine cylinders are not working properly. Driving with a misfiring engine could damage your catalytic converter, which is a very expensive part. Check your owner's manual, it will probably suggest to reduce power and have your vehicle serviced immediately by your authorized dealer. Read more about symptoms of a misfiring engine: Code P030X - cylinder misfire.

What are the common problems that can cause the Check Engine light?

In older cars, it was typically something like a bad oxygen sensor, worn-out spark plugs or ignition wires. Modern cars have a lot more electronics, which means, many other things could go wrong too. It's practically impossible to find the problem without at least scanning the vehicle and retrieving the stored code(s). On the other hand, once you know the code that caused your Check Engine light, it's not that difficult to do some research and find out common problems for your car's make and model that can cause the particular code. From our experience we can name several more frequent problems that can cause the Check Engine light in many cars: vacuum leaks, issues with a mass airflow sensor, failed ignition coils, leaking purge valve or vent valve, failed air-fuel ratio sensor, bad EGR valve and a failed catalytic converter.

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Where to take your car for repairs?

Dealership
Technicians working at a dealership receive regular training from a manufacturer and are familiar with common problems in their cars. They have up-to-date repair information and proper testing equipment, as well as the technical support provided by a manufacturer.

Scanning the car computer for check engine codes
A technician scanning a car


Dealerships use OEM (original) parts and are more likely to stand behind their repairs if something goes wrong. For some trouble codes, the repair involves reprogramming of the PCM with an updated software. Dealers can do it, while most of the independent shops cannot.

It also might be a good idea to visit your dealer if your warranty expired recently. In some cases, manufacturers extend the warranty coverage for certain Check Engine light problems, but they don't advertize this information. Your dealer can check it. If you are a regular customer, or if it's a known problem, you dealer can apply to the manufacturer for a goodwill warranty coverage. We know many cases when the manufacturers cover the full or partial cost of certain repairs in cars that are recently gone out of warranty.
On the downside, out-of-warranty repairs at a dealership tend to be expensive.

Independent repair shop
Independent repair shops are often less pricey, but a lot depends on the professional level of the mechanic, availability of a proper testing equipment, access to latest service information and quality of replacement parts. When it comes to "Check engine" light issues, using proper parts can make a difference between a successful repair and repeated problems.

Brand-specialized independent repair shop
Another popular option is to take your car to an independent shop or a mechanic that specializes in your vehicle's brand. This is especially true for German or other European cars, since they have more complex electronics.

Do it yourself
DIY repairs have never been easier since Youtube became a part of our lives. Thanks to the thousands of automotive enthusiasts sharing their knowledge, there is plenty of info available. If you have sufficient mechanical skills, proper tools and spare time, the first step is to scan your car for codes. Once you know the code, it's not that difficult to read up on common problems causing that code in your make and model. There is a good chance that someone has experienced the same code with the same car like yours and posted the solution. We posted several articles on some of the most common codes:


If you need tools, many parts stores offer to loan tools. Here are few links:
AutoZone Loan-a-tool.
O'Reilly Auto Parts loaner tool program
Canadian Tire Loan a Tool

Where to scan your vehicle for free

If you don't have a scan tool, some auto parts stores and independent auto repair shops offer to scan your car for free, in hopes that you will buy parts or do the repairs at their shop. Here are a couple of links:
Pep Boys
O'Reilly Auto Parts Store Services
Google 'free check engine light scan' + ' your town' to find a shop that will scan your car for free. Some dealers and repair shops offer a free Check Engine light scan as a seasonal promotional. The Volvo Service for Life program, for example, includes up to one hour of computer diagnostics. Another option is to ask your friends and relatives. OBD-II scan tools are not very expensive and widely available. Many people have a scan tool in their households these days.

Emission warranty

A federal emission warranty covers major components of the emission control system such as the engine computer (PCM) and the catalytic converter for the period of 8 years or 80,000 miles (128,000 km in Canada). If your car has the codes related to the failed catalytic converter (e.g. P0420, P0421, P0430) check the emission warranty coverage details with your dealer. Read more about the US Emission Warranty.

How to scan your car for codes yourself

OBD connector
OBD connector, called Data Link Connector or DLC located at the lower portion of the dash on the driver's side

You can buy a decent OBD scan tool for $40-$75 in most auto part stores or online. There are several apps available for smartphones. To connect your phone to the vehicle, you will need an OBD-II Bluetooth or Wi-Fi adaptor. Adaptors like this are not very expensive: $15-$35. How does an OBD app work? You install it on your phone, connect the adaptor to the OBD connector, pair your phone to the adaptor, so the app can communicate with the PCM.

In almost all cars, the OBD diagnostic connector is located at the lower portion of the dash near the driver.

Connecting the scan tool into OBD connector
OBD connector

In this Toyota in the photo, for example, it's positioned near the hood release. In other cars, it could be closer to the center of the dash. This is the OBD connector in the Nissan Pathfinder. This photo shows where the OBD connector is located in the 2009 Honda Accord. The OBD connector is universal in all modern cars.

An OBD scan tool can only scan the PCM, which is the main computer in the vehicle. How it works:
1. With the ignition off, connect the OBD scan tool or a Bluetooth adaptor for the phone app.

Reading codes with a scan tool
OBD connector, called Data Link Connector or DLC located at the lower portion of the dash on the driver's side

2. Turn the ignition ON without starting the car. Follow the menus on the scan tool or the app until you get to the "Read Stored codes" or "Stored DTCs". If your scan tool can access the freeze frame, check it too; it may help in diagnosing the problem. Read more about the Freeze Frame. A scan tool allows you erase the trouble code, but it will come back if the problem is still there.

Check Engine light codes

All PCM codes start with the letter "P" for Powertrain. There are several hundreds of powertrain codes, but some are much more common than others. Check Engine light codes are known as Diagnostic Trouble Codes or DTCs. We wrote about some of the common codes, including possible causes and diagnostic information. Use the scroll down menu.

How to diagnose a trouble code

1. Check Technical Service Bulletins: To diagnose the Check Engine code, the first step is to check for common known problems. Car manufacturers periodically issue Technical Service Bulletins (TSBs) that describe common problems for certain models. When a technician at a dealership or repair shop scans your car and retrieves the trouble code, the first thing he or she does is to check for Technical Service Bulletins. Some TSBs are posted on the internet, and good old Google can help. For example, search for '2014 Jeep Wrangler code P0520 bulletin .pdf' and you will find the Chrysler bulletin in a .pdf format. It says with this code, replace the oil pressure sensor with a revised part. It's a very easy fix.
2. Check for common problems posted by owners and experts: Again, Google can help. Search for the code + make, model and the year of the car. As we mentioned, the same code in the same car is often caused by the same faulty part.
Try, for example, search Youtube or Google for "code P0455 2013 Nissan Rogue"; you will see many articles and videos describing the same problem in this vehicle. Read more about the code P0455.
3. Follow the Diagnostic Flow chart in the Service Manual: The factory service manual contains a list of trouble codes and a step-by-step diagnostic procedure for each code. The service manual is written for skilled technicians and may require use of special tools and testing equipment. If you can get access to the factory service, you can check for TSBs too. We posted the list of websites where you can get a subscription-fee based access to a factory service manual in this article.

Freeze Frame

A freeze frame is a snapshot of data from a number of sensors and components of the vehicle recorded at the time when the PCM detected a fault. The PCM stores the freeze frame along with the related diagnostic trouble code (DTC). If available, the freeze frame can be accessed with a scan tool. The freeze frame can provide a valuable information that can help in diagnosing the problem. Read more about the freeze frame.

Q: Will disconnecting the battery reset the "Check Engine" light?

A: Disconnecting the battery may reset the Check Engine light in most cars, but the light will come back if the problem is not repaired. Also, the readiness code will be erased, which may prevent your car from completing an OBD-II emissions test. The readiness code is an indication that certain emission control components of your car have been self-tested. In cars with the code-protected audio systems, you might need to enter the code to unlock it after the battery is disconnected.

Q: How long a car needs to be driven before it will be ready for an OBD-II emission test?

A: After the vehicle's battery has been disconnected or the Check Engine code has been cleared, a vehicle needs to be driven before it will be ready for the OBD-II inspection (emission test). Typically it takes at least 40-60 minutes of driving including 20-30 minutes of steady highway cruising for your car to be ready for an emission test. In some cases, it may take a day or two of driving. This is because it takes some time for all components of your car's OBD-II emission control system to perform self-tests.

Q: How long does it take for check engine light to reset?

A: If the problem that caused the Check Engine light is repaired, the warning light will turn off. For some faults, it may take just a few minutes of driving. For other problems, it may take a few trips. It takes time for a car OBD-II system to re-test all the components. If the Check Engine light stays on after a couple of days of driving, the problem is still there.

Q: Can overfilling the gas tank cause Check Engine light to come on?

A: Yes, overfilling the gas tank can trigger the "Check Engine" light to come on. Modern cars are equipped with the Evaporative System that prevents gasoline vapors from escaping into the atmosphere. When we overfill the gas tank, the excess gasoline can enter the part of the Evaporative system called the charcoal canister, which is designed to absorb gasoline vapors rather than raw fuel. This can cause problems with the evaporative system that can trigger the Check Engine light.

Tips to prevent the Check Engine light from coming on

• Don't overfill the fuel tank. Fill up till the first click of the pump. Check your owner's manual. After filling up, make sure the gas cap is tight and the gas cap strap does not get caught under the cap.
• Change engine oil regularly. Many Check Engine light problems stem from lack of regular oil changes or driving with low oil level.
• After checking or replacing the air filter, make sure it's installed properly. If unfiltered air is allowed into the engine, it can damage the vehicle's mass airflow (MAF) sensor. It's best to use OEM or known good quality air filters.
• Lack of tune-ups is another common reason for the Check Engine light to come on. Old spark plugs, dirty throttle body, bad ignition wires can cause many problems. Read more: how to maintain your engine.
• The corrosion in the wiring and connectors of the car computer system is another frequent cause of the Check Engine light problems. Your car or truck is full of electronics with wires and connectors located in many places throughout the vehicle, including under the carpet, in the engine compartment, underneath along the frame and other places. Anything you can do to keep the electronic modules and wiring free from corrosion will help avoid related problems. For example, if your engine undercover is damaged, the moisture will enter into the engine compartment where it can cause corrosion.
• When doing repairs, use original parts when possible. Sometimes the Check Engine light comes on because some aftermarket parts like oxygen sensors, mass airflow sensors or catalytic converters can have compatibility issues.

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