Why does the Check Engine light come on?

Updated: September 7, 2014
Check Engine - Service Engine Soon light
Check Engine or Malfunction indicator light (MIL)

It's what almost every motorist has to go through once in a while: the Check Engine 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. 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.

Engine computer or the ECM
The engine computer or Powertrain Control Module (PCM)

To diagnose the problem, your mechanic will have to connect a scan tool to your vehicle and retrieve the code from the computer. 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 did 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 was 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 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 to have 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.

Common problems that can cause the Check Engine light to come on

In older cars, it was typically something like a bad oxygen sensor, faulty mass airflow sensor, failed catalytic converter, worn-out spark plugs, ignition wires, a loose gas cap or a clogged-up EGR system. Newer 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.

Where to take your car for repairs?

Scanning the car computer for check engine codes
A technician at at a dealership using a scan tool

If your car is fairly new, taking it to the dealer makes more sense, as the repairs could be covered by the warranty. Dealers have factory-trained technicians that are familiar with common problems in their cars and have manufacturer-provided technical support. They have specific testing equipment designed for your car and original factory parts. On the downside, repairs at a dealership tend to be more expensive.
Independent or franchise repair shops are usually less pricey, but a lot depends on the professional level of technicians, availability of proper testing equipment, latest service information and quality of replacement parts.
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.
Of course, there is always a DIY option. if you have sufficient mechanical knowledge and proper tools, all you need to start is to scan your car and find out the trouble code (DTC). Thanks to generous people that don't mind sharing their knowledge, there is plenty of information, how-to guides and videos available on the internet. Not all problems can be diagnosed and repaired at home, but that doesn't mean it is not worth trying.

Where to scan your vehicle for free

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. 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. 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 US Emission Warranty

How to scan a car for codes if you have your own scan tool

DLC OBD II Data Link Connector

OBD II Data Link Connector

It's not very difficult to scan your car computer for trouble codes if you have a scan tool or OBD-II software and some technical knowledge.
Step 1: Find the DLC Connector. Any car made after 1996, has a standard diagnostic OBD-II connector, which is in technical terms called Data Link Connector or DLC. The DLC is identical on all modern cars and should be located within three feet of the driver. Usually the DLC connector is located at the lower portion of the dashboard on the driver side, like this one in the photo. In some cars the DLC connector is hidden under a cover, but the connector still looks like the one in the photo (click on the image to enlarge).

Scan tool connected to the DLC

Connecting the scan tool


In some vehicles the DLC connector might be near the fuse panel; in some Acura cars it's hidden under an ashtray. In older Volkswagens the DLC connector is placed under the slide cover at the center console. Some cars have the sign 'OBD' marked on the DLC connector cover.
Step 2: Connect the scan tool:
Once the scan tool is connected, turn the ignition ON, but don't start the engine. Follow the scan tool menus 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. Read more about the Freeze Frame below. Check the scan tool manual for details.

Trouble code displayed on a scan tool screen

Trouble code displayed on the
OBD-II scan tool

Once the fault code is retrieved, more testing will be needed to find the defective part, as the code only indicates the parameter that is out of range. Often a car may have multiple codes. For example, as you can see in this photo, this car has three codes: P0101 - Mass or Volume Air Flow Circuit Range/Performance, P0140 - O2 Sensor No Activity Detected Bank 1 Sensor 2 and P0171- Air fuel mixture too lean. None of the codes actually points directly to the defective part. Upon further testing, we found that dust on the mass airflow sensor caused all three codes. Cleaning the mass air flow sensor solved the problem.

Where can I buy an OBD-II scan tool or software?

Using scan tool software
Using scan tool software

An OBD-II scan tool can be bought at most auto parts stores, or online. A simple OBD code reader may cost anywhere from $25 to $50. A more advanced scan tool with wider capabilities costs from $150 to $350. You can also buy an OBD-II adaptor with the software for your laptop. There is even an OBD-II Android app, but it requires a separate Bluetooth adaptor that plugs into the DLC connector. It's a $10-$20 part that can be ordered online.
An OBD-II scan tool should work on any OBD-II (or EOBD in Europe) compatible car. It's worth mentioning that an OBD-II scan tool cannot be used to diagnose the Airbag or ABS problems; your dealer is the best place to call for those issues.

OBD trouble codes

trouble codes
OBDII trouble codes

The trouble codes on all OBD-II cars are standardized and each code has the same meaning on all OBD-II cars. There could be some minor differences in the way different car manufacturers interpret the same trouble code, but the basic meaning is the same. A typical OBD-II trouble code starts with a letter that is followed by four digits. The letter "P" stands for powertrain, the letter "B" for body. For example, if the engine cylinder number 2 would misfire, the car computer (ECM) would turn on the "Check Engine" light and store the diagnostic trouble code P0302 in its memory. If you'd connect the scan tool, it would read something like: P0302 - Cylinder 2 Misfire Detected. Overall, there are a few hundreds trouble codes for the powertrain, but only about 40-50 codes are very common. You can read more what some of the common powertrain codes mean here: OBDII Diagnostic Trouble Codes

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 that can cause that particular trouble code in this vehicle's make, year and model. Car manufacturers periodically issue Technical Service Bulletins (TSBs) that describe common problems for certain models. When a technician at a dealership or an independent repair shop scans your car and retrieves the trouble code, the first thing he or she is does is to check for Technical Service Bulletins. The TSBs are not freely available, but there are several websites where you can get access to TSBs and other repair information for a fee; see below. You also can use the power of good old Google. For example, search for '2001 V6 Honda Accord code p0401 bulletin .pdf' and you may find a Honda bulletin in the .pdf format that describes the common problem that can cause the code P0401 in 2001 Honda Accord.
2. Check for common problems posted by owners and experts: Again, Google can help. Try for example: code P0171 2004 Ford Explorer and you will find plenty of information including some videos. Similarly, if you have a Honda CR-V with the code P0134, the research will show you that very often the code P0134 is caused by a faulty front air/fuel ratio sensor. We also have many common Check Engine codes listed along with examples and common repairs; check here: OBDII trouble codes. The next step is to use the service manual for your vehicle.
3. Follow the Diagnostic Flow chart in the Service Manual: A manufacturer's service manual contains a list of trouble codes and a step-by-step diagnostic procedure for every specific code. A factory service manual is designed for skilled technicians and may require use of special tools and testing equipment. Check the list of websites where you can access a service manual below.

Freeze Frame

Freeze Frame
Freeze Frame sample. Click for larger photo

A freeze-frame is a snapshot of the engine and transmission parameters at the moment when the engine computer detected a fault and the trouble code was stored. The freeze-frame may show whether the vehicle was stopped or driven at a high speed, whether the air/fuel ratio was lean or rich and whether the engine was cold or fully warmed up at the time of the malfunction. The freeze frame is stored in the engine computer along with the trouble code. How can it be useful? Checking the freeze frame can help identify the problem faster. For instance, if you look at this image, this freeze frame for the code P0116 - Engine Coolant Temperature Circuit Range/Performance indicates that the engine coolant temperature was -40F while the intake air temperature was 84F, which is obviously impossible. The engine temperature should be close to the ambient temperature (intake air temperature) if the car is just started or it should be a lot higher if the engine is warmed up. This means that the engine temperature sensor didn't register the correct temperature. This was most likely caused by either a faulty engine temperature (ECT) sensor or poor connection at the sensor. Looking at this freeze-frame, you also can see that the car was idling at the moment this malfunction was detected (the engine speed showed 756 RPM) and the vehicle was stationary (Vehicle Speed at 0 MPH).

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

A: Disconnecting the battery may reset the Check Engine light in some cars, but the light will come back anyway if the problem is still there. 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.

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

A: After the vehicle's battery has been disconnected or 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 to come on is repaired or no longer exists, the Check Engine 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. This is because 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 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 some problems with the evaporative system that can trigger the Check Engine light.
Don't overfill the tank past the first click of the pump. Check your owner's manual.

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. Overfilling can cause problems with your vehicle's Evaporative emission control systems (EVAP) resulting in the Check Engine light coming on.
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. Long intervals between the oil changes do not directly cause the Check Engine light to come on, but many Check-Engine light problems stem from lack of oil changes.
• After checking or replacing the air filter, make sure it's installed properly. If unfiltered air is allowed into the engine it can cause damage to the vehicle's mass airflow (MAF) sensor, which will turn the MIL on. It's best to use only 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 all can cause engine problems with the Check Engine light coming on.
• The corrosion in the wiring and connectors of the car computer system is another major 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 areas behind the kick panels, under the carpet, in the engine compartment, underneath along the frame, near the gas tank, etc. 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, it can allow moisture into the engine compartment causing corrosion. Similarly, moisture inside the car can cause corrosion of the wiring under the carpet. Keep the electronics dry.
• 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.