Check Engine light: what to check, common problems, repair options

Updated: February 5, 2017
Check Engine - Service Engine Soon light

The Check Engine Light staying on means that there is a problem in your car that affects exhaust emissions. It's a common issue in all cars and your mechanic or local dealership should be able to help you. Is the vehicle safe to drive? Where to take your car for repairs? What are the common problems? We will try to answer these questions in this article, but first, let's see how it works.

Your car has a computer called Powetrain Control Module (PCM) that controls the engine, transmission and emission control systems. There is about 15 to 25 of various sensors that constantly monitor the engine, transmission and other systems. When one of the sensors detects a problem, the computer stores a fault and illuminates the Check Engine light.

The fault is recorded in the PCM as a code. For example, P0301 - Cylinder 1 Misfire Detected. There is a few hundreds of possible codes. The code itself doesn't say what part is bad, it only points to the system or sensor in question.

To repair the problem, first, your mechanic has to scan the PCM and read the code. Then he or she has to perform some tests to find what caused it. There is a separate diagnostic procedure for each code.

There is also a few things that you can check yourself. Start by answering these questions and follow the link.

1. Is the Check Engine light flashing repeatedly? Yes No
2. Is your gas cap closed properly? Yes No
3. Does your engine have enough oil? Yes No
4. Has the Check Engine light came on soon after you had your car has been serviced? Yes No

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

It depends on what the problem is. Some problems are minor and won't affect your vehicle's performance, but in other cases, driving with the Check Engine light may cause more damage to your vehicle. In worst cases, a car may stall or lose power unexpectedly. To be on the safe side, we recommend to have your car checked out as soon as possible.

What are the repair options?

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

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 technical support provided by a manufacturer. Dealerships use OEM (original) parts and generally stand behind their repairs if something goes wrong. Of course, out-of-warranty repairs at a dealership tend to be expensive.

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

Brand-specialized shops
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 generosity of thousands of automotive enthusiasts sharing their knowledge, there is plenty of information, how-to guides and videos available on the internet. If your "Check Engine: light is on, there is a trouble code stored in your vehicle's computer. Once you know that code, it's not that difficult to read up some information on the internet. There is a chance someone has experienced the same problem with his or her car and posted the solution. If you have sufficient mechanical skills, proper tools and some spare time, all you need to start is to scan your car for codes.

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. 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.

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

There are hundreds of potential faults that can cause the Check Engine light, but several problems are common in many cars. According to our extensive research, they include: vacuum leaks, issues with a mass airflow sensor, failed ignition coils, leaking purge valve or vent valve, failed oxygen sensor, bad EGR valve and failed catalytic converter.

The Check Engine light flashing repeatedly

If the Check Engine light is blinking repeatedly, it means that the engine computer has detected that your engine 'misfires', or some of its cylinders are not working properly. Driving with a misfiring engine could damage your catalytic converter, which is an expensive part. If you check your owner's manual, it will probably suggest to reduce power and have your vehicle serviced immediately by your authorized dealer.

Is your gas cap closed properly?

The Check Engine light might also come on if your gas cap is not closed tight. In some cars, the string that holds the cap might get caught in the cap's thread and prevent it from sealing. 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.

Do you have enough oil in the engine?

The low oil level also can cause the Check Engine light to come on. Driving with low oil level can actually damage your engine. Your owner's manual describes how to check the engine oil level in the 'Maintenance' section. Read more here: Easy car maintenance checklist with illustrations.

The Check Engine light came on soon after the car was serviced

If the light came on soon after the vehicle was serviced, it's reasonable to have the same shop re-check your car. Whether the problem is related to the last service or not, it's a common practice for reputable repair shops to assist their loyal customers as much as possible in cases like that. The repair cost will depend on the cause.

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
OBD-II Trouble codes