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)

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.

Oxygen sensor

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.

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.


Can you tell me what may be the problem when the VSC trac light and the check engine light came on at the same time? I have a 2004 Toyota Rav 4.

thx 4 your ideas but when I start my car, the engine check goes off and I can drive almost 300 km but when I stop for few five minutes when the engine is on, the Engine check comes on. what might be a problem? Can a linkage (top cover seal) cause a problem to engine check ?

I gave a warranty on my 07 Monte Carlo as and my car is have problems but every time I take it to the dealer they say there's no problem and tell me there's other things wrong please help.

1996 well-kept Mercedes was taken for inspection. The check engine light was off. When it came through inspection, the light was on and the car was failed. Turning the key off and on reset the light. After a discussion with the techs, we decided I'd drive it for 15 min, then come back and run through inspection again. The light was off. They plugged in their machine, and the light switched on. Amazing. One tech went to get a supervisor. We turned the key off and back on while it was still plugged in, so technically 'it passed'. I have the sticker, but we witnessed the machine turn on the check engine light three times. The techs were good to work with me as we tested the machine. Either the machine is broken, or there is something very strange wrong with the vehicle. LH

I've had 2 new catalitic converters put on, a solenoid, revap purge regulator, a bank 2 oxygen sensor and a new factor gas cap, but my check engine light never stays off. The code is P0441. I've had repairs done by different mechanics. Is it the mechanics or my car?? I'm at a loss. My car is not going to pass inspection once again and I'm afraid to pay more money for a repair that won't work again. Any suggestions?

If your car is running fine and smooth and your gas cap is on tight ignore that light as most of the things that set it off have nothing to do with the actual performance of your engine. Most of it is crap from the smog emmissions in your tail pipe. The dealers love it, as they can sell you a bunch of parts and labor. If your car does not have to be smog checked to register it forget it.

Using a code reader to pull the check engine light code will point you in a direction to look, but it will not tell you precisely what is wrong with your vehicle. In most cases, you will have to have it diagnosed by a competent mechanic. For Instance, you may get a code for a catalytic converter inefficiency which may turn out to be a bad O2 sensor. Also, while spark plugs used to be a fairly easy inexpensive fix, most newer cars require more expensive plugs and may be located in the least-desirable places (ie under the intake). In most cases, I would recommend taking your vehicle to a qualified mechanic.

Pull over and check your oil level.

what is the best OBD

A no cost option is to call a national auto parts store, to see if they offer free testing, using a diagnostic tester (OBD II). You can also purchase an adapter which plugs into your car's OBD II port (usually found under the dash, near the steering wheel) which connects to your laptop computer's USB port. The software comes with the cord, and I paid about $30 for mine, including shipping. You can easily search the internet for more information on OBDII readers and adapters for your PC. It won't fix the problem or make you a mechanic, but you won't have to rely solely on the explanation of the problem which is given to you by your local mechanic or dealership.

Even though draining the battery of your car shuts the check engine light off it doesn't make the problem go away because when you drain the battery and then Try to get a sticker on the vehicle the vehicle will not pass. Although the light is off and you've cleared the memory you cannot beat the system because draining the battery sets all the computers monitors to not ready and the inspection machine picks up on that and the car will get a rejection sticker. Same thing happens when you shut the light off with a code reader. So think twice about draining or disconnecting the battery or using a code reader to get rid of the light because the problem is still there so isn't the light. Get it fixed the proper way.

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