Why did my car's check engine light come on?
The check engine light on your vehicle's gauge cluster can indicate minor or more serious problems. (Photo by Paul Shepherd)
Dealing with a check engine light on your vehicle’s dashboard can be frustrating. While the little light is meant to warn you of a problem – what the problem exactly is can be a mystery. Somewhere inside your vehicle, something happened to trigger the light.
Although you'll probably have to take your car to the shop to get the exact answer, there are a few things you can do immediately after seeing the "service engine soon" light turn on for the first time.
Get your car checked out
Some also refer the check engine light to the service engine soon light and the maintenance required light.
If the check engine light turns on and you start noticing problems with your vehicle's performance, take it to an auto service shop right away. In some cases, a yellow check engine light may indicate that a problem exists, but it's not very urgent. Other times, a red check engine light or a blinking check engine light may occur. In those cases, the problem most likely requires immediate attention. In these cases, it could be a problem related to you car's emissions system or a costly repair like needing a new catalytic converter.
In some cases, it could take time for your mechanic to diagnose the problem. But they should tell you how long they think it will take and how much it will cost.
Check your gas cap
A missing or faulty gas cap can cause a check engine light to turn on. The pressure inside your gas tank is thrown off-kilter when the gas cap is not on or not secured properly. To your car's on-board diagnostic system, or OBD system, a change in pressure could indicate a leak in the emissions system, so the light is triggered to alert you to the problem. Replace the missing cap or tighten the existing one to see if the problem goes away. Give it at least a week. If the light stays on, head to the repair shop or dealership.
Check the code
Inexpensive code readers are available in auto parts stores. If you are the do-it-yourself type, it may be worth it to invest in one. At the same time that your car's check engine light triggers, a code is stored in the memory of its computer. With a code reader, you can pull the code to find out the precise cause of the problem.
Be wary of DIY workarounds
Take check engine lights seriously. Sometimes, people just want them to go away. One popular method is to run out a car's battery, which drains all of the current from the capacitor. Doing this also wipes away the data that's stored in your car's computer, which will make it more difficult to diagnose the problem later.
Check under the hood
Open the hood and look for leaking hoses and frayed wires. Take an especially close look at the spark plug wires. If they are cracked or otherwise showing signs of serious wear and tear, your spark plugs may be misfiring. A simple fix is to replace the old wires.
If you're unable to solve your check engine light problem on your own, don't let it slide. Something triggered it, and ignoring the issue is sure to result in an even costlier repair. At the auto repair shop, a mechanic will use a diagnostic computer or an electronic scan tool to pull the code that caused the light to come on in the first place. The repair shop’s diagnostic may discover issues with one of the following components:
Mass airflow sensor
The mass airflow sensor, or MAF sensor, calculates the amount of air that's coming into the engine so that the computer can add the right amount of fuel. A faulty MAF sensor could trigger the check engine light. It can also cause a car to experience impaired gas mileage, increased emissions or frequent stalling. Avoid this problem by changing the air filter in a timely manner.
There are two to four oxygen sensors in the typical vehicle. The oxygen sensor analyzes the amount of oxygen in a vehicle's exhaust to determine how much fuel has been burned. A car's gas mileage is seriously compromised when it malfunctions. Emissions usually increase too. If you wait and ignore the light though, it could devolve into a problem with the catalytic converter.
The catalytic converter converts carbon monoxide and other compounds into compounds that are safe for the environment. If it stops functioning properly, your gas mileage will suffer and your car may accelerate more and more slowly.
Spark plugs or wires
If you get a check engine light along with slight jolts while accelerating, the spark plugs could be to blame. This is a quick, easy and inexpensive fix. To avoid the problem, replace spark plugs on older cars every 25,000 to 35,000 miles. Replace spark plugs on newer cars roughly every 100,000 miles.
Although it's natural to look at a check engine light with dread, remember that it's there for a reason. Ignore it at your own peril.
Editor's note: This is an updated version of an article originally posted on Feb. 14, 2013.