What Causes the Check Engine Light to Come on in a Car?

R. Kayne

Modern cars are run by efficient computerized systems that constantly monitor the vehicle's performance. When a malfunction is detected, the check engine light (CEL) will come on in the dashboard to let the driver know a problem has been detected. Unfortunately, the light itself doesn't tell the driver what the problem is — it could be anything from a loose cap on the gas tank to a problem with the catalytic converter. Fortunately, when the check engine light is triggered, the engine control unit (ECU) will log one or more codes that can help point mechanics toward the source of the problem.

A car's spark plug.
A car's spark plug.

A check engine light doesn't always mean a big problem or an expensive repair is needed. However, driving a car with the light on could mean that you are doing further damage to the engine, depending on the problem. It's always best to get the vehicle checked at your earliest convenience.

A sticky EGR valve could trigger the check engine light.
A sticky EGR valve could trigger the check engine light.

Loose or Missing Gas Cap

If the vehicle's gas cap is loose or missing, the car's computer can register this as a problem with the emissions system. Gasoline from the tank may evaporate out, lowering the mileage on the car. This problem is usually very easy to fix. If the cap is loose, the check engine light should go off a day or two after it's tightened. Replacing a cap is also very inexpensive; some auto repair shops may even supply a new one for free.

When the check engine light comes on, take the car to a reputable mechanic so he can diagnose the problem.
When the check engine light comes on, take the car to a reputable mechanic so he can diagnose the problem.

Electronic Fuel Injection System Problems

Engines encompass a number of systems that work together to provide excellent performance under normal circumstances. The vast majority of modern cars are powered by an electronic fuel injection (EFI) system, which uses a number of valves and sensors to ensure the correct amount of air and fuel are fed to the piston chambers to create the right combustion mixture. The mass flow sensor and oxygen (O2) sensors are key parts of this system, and a malfunction in either can cause the light to come on. Problems in the EFI can affect other systems, including the catalytic converter, so it's important to fix them right away.

Fuel injection systems supply fuel directly to the cylinders of a vehicle's engine.
Fuel injection systems supply fuel directly to the cylinders of a vehicle's engine.

Worn Out Spark Plugs

Timing of the spark is also controlled electronically. If the spark plugs are worn out or misfiring, they can cause the car to jolt during acceleration. Bad or broken wires can also cause misfires. Both the spark plugs and the wires are relatively easy to replace, and can improve the car's performance.

Vacuum Leak

Another event that is often responsible for tripping the check engine light is a vacuum leak. A hose or manifold might be loose or cracked, which will disrupt the fuel/air ratio in the engine. A visual inspection can help, but in some cases a vacuum leak can be very difficult to locate. Some repair shops offer a smoke test, in which the air intake system and hoses are filled with smoke, and then closely monitored to see where it escapes. Vacuum leaks that can trip the light include a sticky exhaust gas recirculation (EGR) valve, or more seriously, a blown head gasket.

Catalytic Converter Problems

The catalytic converter is part of the emissions system, where it converts the gasses created by combustion in the engine to less harmful exhaust. In cars that are maintained regularly, this part rarely fails. If other parts, like the O2 sensors or spark plugs, malfunction and are not replaced, they can damage the catalytic converter, however. This is a very expensive repair, but the car cannot run without it.

Diagnosing the Problem

There are so many possible reasons for the check engine light to come on, so using a reader to retrieve the on-board diagnostic (OBD) codes can be helpful when the problem is not immediately evident. Even if the light is only on a short time, a code will be stored in the ECU for later retrieval. In most cars, it is easy for a mechanic or even the car's owner to attach the code reader to the appropriate port and get this information. The code can then be used to determine the problem.

On-board diagnostic codes can be used to discover the issue with the engine.
On-board diagnostic codes can be used to discover the issue with the engine.

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Discussion Comments


I have a 2003 Pontiac Sunfire. I had a new battery put in 2010, then had my car shipped and did not get it back for a month. Ever since, I have been having problems. It cost $700 when I had some wires replaced and the gas tank. I think that is what mechanic said. I had to take the car in because it just would not start. It lost all power. Now the check engine light is on again and stays on. I had the gas cap replaced but the light remains on. What can possibly be wrong? I’m not taking it to the garage a second time. I hardly use my car but I seem to be replacing parts all the time. I don’t think the last garage had a diagnostic machine and I paid for this. What could be wrong? I’m going broke.


Can a bad transmission trigger a light to come on? Always? Or sometimes?


If you look online for your car's fault codes, there are ways to connect certain connections in your fuse box and cause your engine warning light to dash/dot to give you a code which you'll need to write down and look up. They are all numbered and dashes and dots will be a number that correlates to the fault the ECU has detected. Well it works for my Vauxhall (Holden, Opel) Corsa b.


I was driving my car, a Suzuk Kei. When I took it from the selling yard, twenty minutes later, the car started losing the acceleration, the empty tank light showed up on the dashbody and then stopped. There was an adequate supply of fuel. I remained there for some minutes but when I cranked the engine again, it accelerated but after ten minutes, it stopped again.


I have a Chevrolet Epica LT 2004. It is showing the codes P0171 and P0174. I have replaced the mass air flow sensor, PCV valve reset and check engine light. After driving 5 km the check light pops on. What more can I do? Can anyone help?

I know that some car parts shops will run the codes on your car computer and tell you why your check engine light is on for free. This is a really valuable service because it gives you some idea of how quickly you need to get your car into the shop. Some problems require immediate attention and others you can wait on for a week or two.

If I need to get a brake job will that cause my check engine light to come on? The light came on for me recently and I know that the breaks are bad so I am hoping that I only have one problem to deal with and not two.


I just paid this shop $700 and my check engine light is still on. Now they want to charge me $800 more to install a new timing chain because they say this might be the problem. What do you think? I have a 2001 xL7 with 214,000 miles.


I have a 1997 holden astra. It was parked for three hours then I went to go home. the check engine light was blinking and it won't start. Can you help? The cam belt is not broken.


The check engine light started to flash on and off as i was leaving the supermarket. I drove out of car park about 30 secs down the road and pulled over and switched car. my car was driving beautifully, same as always. I looked in my manual book for 2001 holden barina car to see what the light was all about and it read engine light.

I turned my car back on and there was no light or flashing. I have been into town and made 4/5 trips and still no light or flashing. Is there a problem or is it all OK. Does this happen often? I am going away thursday for a 5 hour drive. Should I be driving my car it is driving the same as always -- smooth with out any problems. -- Karen


I own a Hyundai Accent 2002, Engine 1.6, I´m asking if the timing belt is broken, the Check Engine light

should go on or not?


I notice my engine light is on. what could be the cause?


An easy "check" is to fill your tank and make absolutely sure that the gas tank cap is on properly. If this is the culprit, light goes off after about 20 miles of driving.


just had major work on my car (fiat punto model 2002) and my check engine light came on and off now, but could the new parts in place caused it. i had the head gasket, manifold gasket, cam belt sensors and part my exhaust replaced.

yes i know a lot and over 500 pounds in repair (about 750 dollars). basically i just wondered if this would have been a cause for it to flash and go again?


1997 mirage check engine light on. what could be causing it to be on? the car is still running.

check engine light came on them i put antifreeze and it turned off, then it turned back on.


my check engine light is on. what can i do?


today there started to be a kind of acid like smell, and after it had parked for a while i started driving again and the lights would start to dim when i put the brakes on. after driving it for about 15 minutes the check engine light came on and won't go off, any thoughts? maybe a leaking battery? and if so why isn't the battery light on?


my car has a unstable idle and tpm register an increase when driving. Could this be the cause of a bad injector? my check engine came on also.


How long are codes stored in a 2007 Hyundai Accent's

ECU after a light that has been on for a while turns off?

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