Check Engine Light 101

So your Check Engine Light (CEL) came on. For a lot of people this light is a mystery and hints at some unseen danger in the guts of your car. Others ignore it until it goes away, or just let it stay on.

The Check Engine Light (also known as the Malfunction Indicator Light) exists to notify a driver that the engine computer has noticed a problem and requires attention. The on-board diagnostics system and engine control unit are in charge of monitoring a bunch of things like car emissions, and if they get a signal that’s abnormal, it triggers the CEL.

We get a lot of questions about the CEL in the Automatic customer community. There is some uneasiness surrounding this indicator that we believe stem from a few misconceptions.

Our goal is to make sure you’re informed and that you are able to have a good conversation with your mechanic if you need to. Let’s try to clear some of these up!

Misconception: “A code reader tells you the exact problem.”

[caption id=”attachment_543" align=”aligncenter” width=”274"]

While a DTC tool can give you an idea, you should still check with a mechanic.

While a DTC tool can give you an idea, you should still check with a mechanic.[/caption]

A reading of a CEL trouble code can be confusing because the trouble code is only one piece of information, a single symptom of a possibly larger problem. Imagine going to WebMD with a headache, and trying to figure out what it means — a headache can mean anything! If it’s bad enough, you probably need to go see a doctor. This is the same for your car. Engine codes should be diagnosed by a professional, but identifying and understanding the symptoms can help you be comfortable talking with your mechanic.

If you’d like to know more about codes you might see, here’s a FAQ that discusses codes in depth.

Misconception: “Clearing the light clears the problem.”

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Clearing the light over and over might get you into real trouble.

Clearing the light over and over might get you into real trouble.[/caption]

When the CEL comes on, some technicians will clear the light once before attempting to fix the problem. This is fine for you to do, too. It’s possible that the light was triggered accidentally. If the light comes back on, figure out what the problem is and fix it if you can. Clear the light and check it again. However, if it comes back on, it’s time to call the mechanic.

Some people like to continually clear the light and put off checking in with a mechanic. This isn’t your best bet because ignoring the problem might make it worse — sometimes substantially.

Make sure you can retrace your steps: what code did your scanner report? What did you try to do to fix a problem? Did it work? Keeping track of your steps will be useful when talking to your mechanic.

You can read more about this topic in the Automatic community.

Misconception: “The CEL gives information related to all problems with your car.”

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Check engine lights only tell you one thing: something's up and you should check it out.

Check engine lights only tell you one thing: something’s up and you should check it out.[/caption]

This is not the case, and that’s actually a good thing! The CEL gives you just emissions related issues. Lights related to airbags, maintenance, ABS, tire pressure, and car batteries are different from the CEL. If you had to rely on one light to tell you all your problems, it’d be annoying to figure out where to begin. With the different lights on your dash, you can at least know where to start looking.

These are some of the misconceptions, but not all of them.

Don’t fret if your CEL comes on*, just make sure you know what’s happening inside your vehicle as soon as you can. You’ll need a scan tool/code reader (like Automatic) to give you the rundown.

*Note: If your CEL is blinking while your engine is running, it means you’re doing “mechanical damage” to your vehicle if you continue to drive under these conditions. If this happens, you should seek immediate professional advice.

Looking for more resources? We can help. Ian, our resident automotive engineer, also suggests checking out these resources if you’d like to learn more about CEL/MIL or OBD-II.

Any other questions? Ask us!