Why Your ‘Check Engine’ Light Is On

Why Your ‘Check Engine’ Light Is On
Gasp! (Photo: Getty Images)

You’re driving home from work one day when the check engine light pops on. It comes on without warning and offers no explanation. What do you do?

As Jalopnik has pointed out, the check engine light is one of the most frustrating and confusing warnings. It’s just a light, with no information telling you what the problem is. It’s a cry from a baby with no explanation. But you can take some steps towards figuring it out yourself, before you head to a mechanic for costly repairs.

Do this first

First of all, is the light blinking or steady? According to Consumer Reports, blinking could indicate an emergency. If the light is blinking, pull over, turn the car off, and call for assistance.

A steady light means there’s no immediate cause for alarm, but you should still get it checked out. Head to an auto parts store and have them run a diagnostic to find the cause of the check engine light. Call ahead and make sure they can handle your make and model, since some cars have special computers. Once you’re at the store, they’ll plug a small computer underneath your dashboard and read back a code stating what happened to the car. (You can also purchase your own code reader, known as an OBD II reader.)

In general, five common malfunctions can cause the check engine light to come on, and some of them are simple to fix yourself. Let’s take a look at the most common issues.

Oxygen sensor needs replacing

An oxygen sensor is a part that monitors the unburned oxygen from the exhaust. It helps monitor how much fuel is burned. A faulty sensor means it’s not providing the right data to the computer and causes a decrease in fuel mileage. Most cars have between two and four oxygen sensors; the code you get from the scanner will tell you which one needs replacing.

What causes it

Over time, the sensor gets covered in oil ash and it reduces the sensor’s ability to change the oxygen and fuel mixture. A faulty sensor not only reduces gas mileage, it also increase emissions.

What you should do

Not replacing a broken oxygen sensor can eventually lead to a busted catalytic convertor which can cost upwards of $3000. Taking your car into a shop will cost you around $200-$300, depending on the car. However, an oxygen sensor is easy to replace on many cars and is usually detailed in the owner’s manual. If you know where the sensor is, you only have to unclip the old sensor and replace it with a new one. Regardless of how you approach it, get this fixed right away.

You have a loose or faulty fuel cap

ImageChris Chan” loading=”lazy” > Photo: Chris Chan

You wouldn’t think a fuel cap would be that important, but it is. When it’s loose or cracked, fuel vapors leak out and can throw the whole fuel system off. This causes a reduction in fuel mileage and increases emissions.

What causes it

If you get an error pointing to the cap it means fuel vapors are leaking out of your cap. This means the cap is either cracked or just wasn’t tightened well enough.

What you should do

If your car isn’t feeling jerky or strange when the check engine light comes on the first thing you should check is the fuel cap. Pull over, tighten it, and take a look at the cap to see if it has any cracks in it. Continue driving and see if the check engine light turns off. Alternately, you can purchase a new cap for a few dollars online or at an auto parts store. All you need to do is take the old one off and screw on the new one. If you’ve already made it to the store, you might as well replace it. While not car-threatening, it’s good to take care of this right away to improve gas mileage.

You need to replace the catalytic convertor

ImageNoricum” loading=”lazy” > Photo: Noricum

The catalytic convertor works to reduce exhaust gases. It converts carbon monoxide and other harmful materials into harmless compounds. If your catalytic convertor is failing, you’ll notice a decrease in fuel mileage or your car won’t go any faster when you push the accelerator.

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What causes it

Catalytic convertors shouldn’t fail if you’re keeping up on regular maintenance. The main cause of failure is related to other items on this list, including a broken oxygen sensor or deteriorated spark plugs (we’ll get to those in a second). When it fails, it stops converting carbon monoxide into less harmful emissions.

What you should do

If your catalytic convertor fails completely, you eventually won’t be able to keep the car running. Your fuel mileage will also be terrible, so you should try and fix it as soon as you can. Unfortunately, a new catalytic converter can cost you a couple thousand dollars and you can’t do it yourself unless you’re an experienced mechanic.

You need a new mass airflow sensor

The mass airflow sensor tells the car’s computer to add the proper amount of fuel based on the air coming through to the engine. A faulty one can increase emissions, cause the car to stall, and decrease fuel mileage.

What causes it

Most mass airflow sensors fail because of a improperly installed (or never replaced) air filter. You should replace the air filter at least once a year to help prevent the airflow sensor from failing.

What you should do

Theoretically you can drive for a few weeks or even months with a broken MAF sensor. You will notice a decrease in fuel mileage and over time the car will eventually start stalling a lot. At a shop, the replacement cost is between $300-$500 depending on the car, but that’s usually the cost of parts because the labour is relatively simple. It’s not terribly difficult to do on your own, but the process is quick enough you may want to let a mechanic handle the project in case the sensor doesn’t end up being the issue.

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You need new spark plugs and wires

The spark plug seals the combustion chamber and provides a gap for a spark to jump across and initiate combustion in your engine. When the plugs are failing, the spark plugs misfire. You’ll feel a little jolt in your car’s acceleration when this happens.

What causes it

Most spark plugs in cars from before 1996 should be replaced every 40,000 kilometres or so. Newer ones can last up to 160,000km. Still, plugs fail over time and there’s not much you can do about it.

What you should do

Get them replaced right away. It’s easy and cheap and your car will run better for it. If your spark plugs fail and you’ve made your way to the auto parts store, you can replace them easily following the video above. Since this is part of your vehicles regular maintenance, the spark plugs are usually easily accessible from the hood of the car. It’s simple enough that I’ve seen people doing this in the auto parts parking lot on several occasions.

Plenty of other possibilities for a check engine light are out there, but the above five are the most common. How long do you usually let the check engine light stay on before you do something about it?

This article has been updated since its original publication.


  • I literally had the check engine light problem a week ago. Unfortunately it was at the worst possible time, I’d just got back from overseas and retrieved the car from long term parking at 11PM on a Sunday night with a couple friends with me. Go to leave the parking lot and the check engine light comes on and the car goes into limp mode (top speed somewhere between 5-10kmph). 3:30am before I got home in a tow truck.

    Which brings me to the suggestions;

    1. Sometimes check engine/limp mode is a transient fault. So you should do the IT Crowd thing and turn if off and on again. You can take that a step further and try disconnecting the battery for 5-10 minutes as that clears the error codes in the computer all modern cars now come with.

    2. In my case that didn’t work so the next step is calling the RACQ (or whichever auto-assistance body you have in your state). Make sure you tell them exactly what’s going on. The callout mechanic will typically have an OBDII reader and they can check and potentially reset or fix the error. Failing that they can at least organise a tow. And to be honest, if your car is in limp mode (rather than just showing the warning light) you’re probably better just gettting RACQ to organise a tow straight away.

    In my case I needed to replace the EGR (exhaust gas recirculator) and it wound up costing just shy of $1000. With subsequent checking I found I could get a replacement unit for about $200 on ebay (though I’m not sure it was genuine) and could have fitted it myself.

    So make sure you get the error codes from the RACQ mechanic and google them before you actually take your car to a mechanic. The EGR unit (mine at least) has a part number on it which I could google to find the exact item needed. Even if you can’t fix it yourself you may be able to supply the part and save costs that way.

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