Check Engine Light On

March 6, 2018

If your check engine light comes on, it can give you a feeling of dread. Is something about to go wrong? How much is it going to cost? Don't panic — let's look at why the light might be on and what to do about it. Take the quiz or keep reading below to get an idea of what might be wrong.

The check engine light illuminates when your vehicle's computer believes there is a problem that could affect — or is affecting — your car's emissions control system.

Check Engine Light Check Engine Light 

When the light comes on, one or more diagnostic trouble codes are stored in the car's computer. These codes will stay there even if the light goes out. To address a check engine light issue, you or your mechanic can retrieve the codes by using an OBD-II scanner and then troubleshooting to find the problem.

If your light is on, it typically means the car's emissions control system is faulty and the vehicle is polluting the air beyond allowable federal standards. 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.


What to do when your check engine light comes on

Not all check engine light warnings are created equal. Here are four common scenarios, and what they mean:

1. The check engine light comes on only in certain conditions, or flickers.
If the check engine light comes on in the city but goes off on the freeway, then the problem is only happening in city driving conditions. It also might flicker inconsistently. Pay attention to whether or not the vehicle runs or drives any differently when you see the light. If you notice a difference, drive the car as little as possible and take it to your mechanic. If there's 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.

2. The check engine light comes on and stays on.
If the check engine light is on 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 emissions system will usually make it so your car runs only in a few lower gears — this is known as "limp-home mode." You should get the car serviced as soon as possible.

3. 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 many cases, the vehicle is not safe to drive at all — it could stop or stall out at any moment. It's best to pull over to a safe place and have the vehicle towed to a repair shop for a thorough inspection.

4. The check engine light comes on and blinks in a steady pattern while driving.
Don't confuse this steady pulsing of the check engine 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. 

Common causes of a check engine light

A number of problems can cause your check engine light to come on, but some are more common than others. To properly identify your issue, consider consulting a mechanic or purchasing an ODB-II code reader.

  • Gas cap is loose: Tighten the gas cap. If you've lost it, replace it with a new one.
  • Oxygen sensor is bad: You'll need a new sensor — but identify which one is bad first.
  • Spark plugs may be fouled or failing: Replace the spark plugs.
  • Spark plug wires are bad: Try replacing the spark plug wires.
  • Catalytic converter is faulty: Have a mechanic inspect and possibly replace the device.
  • Mass airflow sensor is bad: You'll need a new sensor.

About the Author

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28 User Comments

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By , June 27, 2010
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.
By , July 16, 2010
By , July 21, 2010
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?
By , January 03, 2011
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.
By , January 10, 2011
is it safe to drive about 2 miles when ECM is damage on civic
By , January 22, 2011
Thanks, yes it was my gas cap. However how do you turn off the engine light? Do I have to go to a shop?
By , February 26, 2011
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?
By , March 01, 2011
Just went and retightened the gas cap and then turned on engine and the check engine light turned off.. Thanks..
By , March 01, 2011
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....
By , March 22, 2011
just use some scan tool/diagnostic tool to turn off the check engine light.
By , March 23, 2011
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
By , March 27, 2011
my gas cap was on wrong.
By , July 30, 2011
had this checked the code # p0240 what is this mean
By , July 30, 2011
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.
By , June 15, 2017
I have a 2007 ford f-150 XL with a 4.2 L engine. I had it tuned with new coils and plugs. The check engine light is still on. I can drive it and it is fine but sometimes when I have to come to s stop at a light it will stall out. It will start up ok but will stall out again usually at another stop. The exhaust sounds like it has back pressure when I gun it. Do I need a new muffler to eliminate the back pressure issue? Or is it something else?
By , June 21, 2017
I had the same situation at about 150k mi (just went over 200k!) I was chasing switches and connectors, Throttle body position sensor, all four O2 sensors, coil rack, camshaft position sensor, engine coolant temp switch, cleaned injectors and fuel line, valve cover line to the throttle body area, etc. Every thing that could be replaced except for the camshaft sensor that cannot be swapped out. I think it's behind the trans bell housing and firewall. It seems that all of the items mentioned above start failing at 150k. I didn't feel to bad about the cost involved, because it's all good now, and no probs for 50k. I found that once a particular sensor goes bad, the PMC makes adjustments to help alleviate the problem, and that might include automatic adjustments to the #3 cylinder injector because it thinks the fuel mix is too rich or lean. It won't run normally anymore because it goes downhill trying to get back to "normal". Do you recall if the check engine flashed on and off at anytime? When flashing it's saying stop right now, this very instant and repair me , or I'll make your life hell. The flashing light goes out after awhile and it's back to just the non flashing CE light. If you continue to drive after that, thinking things are back to normal, you've just fried the catalytic converter and they need to be replaced. Hopefully all the components that you installed will still be good. Clean out the TB intake area. It's probably dirty all hell. Check vacuum lines too. Here's the kicker: While I don't advocate using chemicals to clean inside the engine (most of that junk will eat gaskets and o rings, etc ), I did use "Seafoam" as a cleaning agent, and it actually worked. Take your jeep out to some very rural place, or behind Target, etc. Because after it starts working It's going to look like you've started a bonfire of plastic. Thick black smoke. The procedure takes about 10 minuets, and that should give you the time needed to get outta there before the fire trucks arrive. I might have missed a thing or two because it was 3 years ago for me,
By , July 17, 2017
What about a bent solanoid and leak in the gas cap
By , August 06, 2017
P0019 Points to the Crankshaft sensor on a 2014 Dodge Journey 3.6 Diagnostic test. Replaced the Crankshaft Sensor and both Camshaft Sensor. Still the Vehicle is putting out the same code. Is there a relay between these sensors and the Computer?
By , August 14, 2017
coils replaced ,was told it may be the module
By , August 17, 2017
Hi. My 2006 Toyota hybrid highlander displays check hybrid system on the dashboard. What do you suggest is the likely fault
By , August 17, 2017
My Toyota hybrid highlander 2006 displays chech hybrid system on the dashboard. What do you suggest is the problem
By , September 17, 2017
Have been having problems for over a month with "check engine" light. Only comes on when engine is cold. Engine runs fine after warm up. Last week or 2 "check engine" light flashes on start up only when engine is cold . Car runs normal after warm up. What is the cause?
By , September 20, 2017
Vent valve solenoid needs to be replaced.
By , January 02, 2018
VW 2010 Passat B6, Misfire and vacuum leak
By , March 02, 2018
2005 gives me trouble starting and after it starts I can pump the gas petal and itcacts like its not getting any gas and stalls out
By , May 07, 2018
2 shops with 2 different diagnosis one is the computor the other says throttle body, 2006 GMC ENVoy sle suddenly started running rough not driveable
By , May 10, 2018
My check engine light came on and the code that was giving is p0712 tramnsmission temperature sensor so I go to Toyota and they sell me a part ,it's the wire transmission cable with 6 sensor leads and 8t goes inside the transmission. I'm like we're and we're does the clip come out to connect. Can somebody please help me to locate the main clip that connects to the wire harness coming out of the transmission
By , February 05, 2019
2002 Ford Explorer.. Check engine light on steady now.. code says running lean or oxygen does have an exhaust leak but it never caused the light to come on before.