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

Updated: February 5, 2017

You have enough problems to deal with already in your busy life and now your car gives you one: the Check Engine light stays on. It's the source of frustration of many motorists around the world, but, same as you did with many other problems, you can solve this one too.

Check Engine  symbol

First, let's see what it means. The Check Engine light comes on when the central computer in your car called PCM detects a fault that can affect exhaust emissions.

How it works: your car has 15 to 20 sensors that monitor your vehicle performance and emissions. For example, there is a mass air flow sensor that measures the amount of air that enters your engine. There is an air-fuel ratio sensor that measures the amount of oxygen in your exhaust, and so on.

When one of the sensor detects something abnormal, the computer sets the code and turns on the Check Engine light. There are few hundreds possible codes. The code doesn't indicate what part to replace, but rather points to a sensor or system that malfunctioned. Therefore, the first step to diagnose the Check Engine light is to read the code stored in the computer.

There are also a few basic 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. Do the battery terminals look badly corroded or loose? Yes No
5. 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 having 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. 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.

Advertisement - Continue reading below

How to scan your car for codes yourself

Scan tools are widely available and not very expensive. You can buy a decent OBD-II scan tool for $40-$75 in most auto part stores or online. There are several apps available for smartphones, but to connect your phone to the vehicle you will need an OBD-II Bluetooth or Wi-Fi adaptor.

Scanning the car computer for check engine codes
OBD-II connector, called Data Link Connector or DLC located at the lower portion of the dash on the driver's side

How it works: All modern cars have an OBD-II diagnostic connector located somewhere near the driver. In most cars the connector is at the lower portion of the dash on the driver's side. See the photo. Any OBD scan tool connects to a vehicle using this connector.

An OBD scan tool can only scan the PCM, which is the main computer in the vehicle. Connect the scan tool, turn the ignition ON, go through menus and read the code or codes. Once you know the code, it doesn't take much to google what it means for your car. Using a scan tool you can even erase the code. Of course, the code will come back if the problem is still present.

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 or air-fuel ratio sensor, bad EGR valve and failed catalytic converter.

Check Engine light codes

The Check Engine light is controlled by the PCM, so 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. Scroll down the menu for a particular code.

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. When the Check Engine light comes on, the PCM stores the diagnostic trouble code (DTC) and the freeze frame related to the code. If available, the freeze frame can be accessed with a scan tool. Read more about the freeze frame.

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 might suggest to reduce power and have your vehicle serviced immediately by an authorized dealer. Read more about misfire diagnostic and repair: Code P030X - cylinder misfire.

Is the 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 another problem.

Low engine oil level

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 here: Easy car maintenance checklist with illustrations.

Battery terminals are badly corroded or loose

A loose or badly-corroded battery terminal is one of the common problem causing the Check Engine light. When there is no solid connection between a battery and one of the cables, the battery voltage could intermittently drop below safe limit, triggering the fault in the PCM or other control modules.

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

You might also be interested: