Check Engine Light On

The Check Engine Light (CEL) is a warning indicator—it means your vehicle's computer has determined that a component or system in your emission control system is not working properly.

Check Engine Light Check Engine Light 

When the light comes on, one or more diagnostic trouble codes (DTC) are stored in the engine control module. These DTCs remain even if the light goes out. To address a Check Engine Light problem, the DTCs are retrieved and the appropriate troubleshooting information is followed in order to determine the problem.

Every vehicle manufactured in the U.S. has to first pass an Environmental Protection Agency (EPA) test called the Federal Test Procedure. This sets the acceptable limits of wear and/or failure for the emission control system—i.e., what conditions will ultimately cause a Check Engine Light to illuminate. These standards are closely regulated. If the emission control system is faulty and the vehicle is polluting the air, the Check Engine Light illuminates to alert the driver of this condition. (Note: A vehicle in this condition would fail an emissions inspection or smog check.)

Don't confuse the Check Engine Light with the maintenance or service light. These lights illuminate when a routine service is due. They are usually triggered by mileage, gallons of gasoline consumed, or some other type of vehicle-use measurement.

The Four Most Common Check Engine Light Scenarios and What to Do

The Check Engine Light is blinking or flickers.
If the Check Engine Light comes on in the city but goes off on the freeway, then the fault is present during city driving conditions. Pay attention to whether or not the vehicle runs or drives any differently when the Check Engine Light illuminates. If vehicle performance does change, drive the car as little as possible and take it to be checked by a service professional as soon as possible. If there is no change in vehicle performance, you can drive home, but have it inspected as soon as possible. In this condition, you run a risk of the vehicle dying or not starting.

The Check Engine Light comes on and stays on.
If the Check Engine Light illuminates constantly during driving with no noticeable driving or performance problems, there is a permanent fault in the emission control system. When this happens, the computer that controls the emission system usually has a backup program that runs while the fault is present. (These backup programs are often referred to as "limp home" mode programs.) You should get the vehicle serviced as soon as possible, but in most cases, the vehicle will continue to operate, though you run a risk of it dying or not starting.

The Check Engine Light illuminates, stays on, and there are performance problems.
This means that a vital component of your emission control and engine management system has a serious problem. It usually involves a component or system needed for the vehicle to run at all. In most cases, drive the vehicle as little as possible. In many cases, the vehicle is not safe to drive at all—it could stop or stall out at any moment. It is best to pull over to a safe place and have the vehicle towed to an automotive diagnostician for a thorough inspection and repair.

The Check Engine Light light comes on and blinks in a steady pattern while driving.
Don't confuse this steady pulsing of the Check Engine Light light (usually one or more flashes per second) with a flicker (see above). The Check Engine Light may stay on steadily or it may flash when the vehicle is accelerated. This is very serious. There is a severe failure of the emission control system that is causing the engine to misfire to the point that the catalytic converter is damaged each time the Check Engine Light flashes. It may mean that the catalytic converter is overheating to the point that it will glow red or, in extreme cases, start a fire on the underside of the vehicle. Immediately pull over to a safe place and have your vehicle towed to an automotive diagnostician for repair. Vehicles can be severely damaged and even destroyed by fire if this condition is ignored for too long.

Common Causes of a Check Engine Light

The following are common causes of a check engine light. In order to identify your issue, please consider a RepairPal Certified shop or purchasing an ODB code reader to identify the issue yourself.

Issue   Possible Solution
Gas cap is loose   Tighten the gas cap
Oxygen sensor (O2) may be bad   Consider replacing the oxygen sensor
Spark plugs may need replacing   Replace spark plugs
Spark plug wires may need replacing   Consider replacing spark plug wires
Catalytic converter may be faulty   Check to see if catalytic converter is faulty or missing
Mass Airflow Sensor (MAF) is faulty   Consider replacing your mass airflow sensor

14 User Comments

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If You look at the fuel cap ,You will see sign-check engine lite will come on if it is not tight.The cap is part of emission system.So if Your car is in good condition and check engine light comes on check fuel cap o-ring it could be all cracked and for computer it is a leek in emission system.Replace 0-ring(You can find proper size on ebay).Second -never file up Your car while engine running,because it dangerous and because the computer in Your car doesn't know what happen with emission system and counts it as a leek.Good lock.
Hey can someone help me with my car? I drive a Ford Mustang 1993. My check engine light recently just turned on. Recently as in about 1 month ago. It used to turn on only sometimes when i drove but it would eventually turn off.. I don't know the reason for that. Later - which is now, Almost everytime i drive it goes on after 5 minutes. Nothing changes about the performance either. is there a list of things for my condition that would of triggered the CEL?
One of the most common myths in diagnosing a check engine light is that a DTC or diagnostic trouble code directly condemns a component- NOT true. P0102 for example refers to the mass air flow sensor. The pinpoint test lists 10 different causes/ components that could trigger this light. One could throw a dart and have a chance to be correct (a costly decision) or perform a pinpoint test and repair the vehicle correctly the first time.
is it safe to drive about 2 miles when ECM is damage on civic
Thanks, yes it was my gas cap. However how do you turn off the engine light? Do I have to go to a shop?
My "check engine" light has been going on & off since summer 2010,without rhyme or reason.I thought maybe the weather (no), speed (no), uphill,downhill (nada). 2 checks when ck eng lite on says it's the knock sensor, but there's been no change in car performance, or mileage. Could it be a loose connection?
Just went and retightened the gas cap and then turned on engine and the check engine light turned off.. Thanks..
I have a 1999 mercedes benz c230, engine light comes on, ive had to replace battery twice in 2 months already, i was told it could be the alternator...ughh....
just use some scan tool/diagnostic tool to turn off the check engine light.
2 0 0 6 Chev Trail Blazer , C E L stays on , My trusted repair shop CLEARED CODE . 1 st time analyzer said , throttle body sensor . Price was decent , but, no money at the time . I adjusted my driving to what car wanted . C E L is still on , 2 years later , PRICE HAS GONE UP 2 HUNDRED DOLLARS FROM ORIGINAL QUOTE . I am going to use chemical throttle body cleaner , as recommended and continue to live with it . Chemical says it is safe for all sensors . T i m
my gas cap was on wrong.
had this checked the code # p0240 what is this mean
After reading your article I went out to my 05 trailblazer....started up and hit the onstar button. They determined after only five minutes that my thermostat was stuck in the open position and recommended a change......problem solved.....engine lite out. Ed M.....Manorville N.Y.