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

Updated: August 9, 2015
Check Engine - Service Engine Soon light

It's the source of anxiety faced daily by many motorists: the Check Engine Light. It means that the computer in your car has detected a problem with one of the systems that affect exhaust emissions. How expensive is the repair? Is it a minor glitch or serious problem? Is it safe to drive? To find the answer to these questions you will need to have your vehicle scanned and properly diagnosed.

It's not that difficult, many car dealers and repair shops deal with these problems every day. There are stores that may offer to scan your car for free. There are also a few things that you can check on your car yourself, but first, let's see how it works.

Your car has a computer called Powertrain Control Module or PCM that controls your vehicle's engine, transmission and emission control systems. There is a number of electronic sensors that monitor various systems. If the signal from any of the sensors moves out of expected range for a certain period of time, the PCM detects a fault, turns the Check Engine light on and stores the code related to the fault in its memory.
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 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 fairly common. They include vacuum leaks, mass airflow sensor issues, misfiring, catalytic converter problems, Evaporative system (EVAP) leaks, oxygen sensor issues and Exhaust Gas Recirculation (EGR) system problems. Read more: Common problems that can cause the Check engine light.

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
Read next:
Common Check Engine light problems
OBD-II Trouble codes