What Does Your Check Engine Light Mean?

The check engine light — more formally known as the malfunction indicator lamp (MIL) — is a signal from the car's engine computer that something is wrong. The light could be a minor issue, such as a faulty gas cap, or it could mean something more serious, such as a misfiring engine. In many cases, it means that you'll be visiting the car dealer to repair the issue and get the light turned off.

Check engine lights come in orange, yellow or amber, depending on the manufacturer. If the light begins flashing, however, it indicates a more serious problem, such as a misfire that can quickly overheat the catalytic converter. These emissions devices operate at high temperatures to cut emissions but can pose a fire hazard if faulty. In this article, we'll cover the top reasons why a check engine light comes on and determine a course of action for this scenario.

1. Most common reasons why the check engine light comes on
2. How to decipher the check engine light with a code reader
3. Possible false flags from a check engine code
4. Don't ignore that light
5. How to turn off the check engine light
6. Brief history of the check engine light
7. FAQs

Most common reasons why the check engine light comes on

CarMD, an automotive telematics company, publishes an annual Vehicle Health Index that compiles the 10 most common check engine codes, along with their estimated cost of repair. For 2023, CarMD's top 10 check engine codes are:

1. Replace catalytic converter(s): $1,313.46
2. Replace oxygen sensor(s): $242.34
3. Replace ignition coil(s) and spark plug(s): $392.65
4. Replace mass airflow sensor: $303.61
5. Inspect for loose fuel cap and tighten or replace: $24.84
6. Replace evaporative emissions (EVAP) canister purge control valve: $137.47
7. Replace ignition coil(s): $213.70
8. Replace fuel injector(s): $423.71
9. Replace thermostat: $238.58
10. Reprogram powertrain control module (PCM): $109.22

Occasionally, the check engine light comes on when nothing is wrong with the car, said Steve Mazor, retired chief automotive engineer for the Auto Club of Southern California. It could be a temporary problem caused by a change in humidity or other factors. In such cases, the light should go off by itself after a short time.

How to decipher the check engine light with a code reader

Some drivers might confuse the service required or maintenance required light on the gauge cluster for the check engine light. These warning lights are unrelated. The service required light just means the car is due for an oil change or other routine care. It is not an indicator of trouble like the check engine light is. Your local mechanic can usually diagnose the problem for about an hour of labor.

But there's a way to preview what the problem might be. Do-it-yourselfers can buy code readers from an auto parts store or online that connect to the onboard diagnostics (OBD) port and search for the code's meaning on websites such as Engine Light Help. Modern systems will display the code in an app on your smartphone.

Mazor says that an inexpensive check engine code reader could be useful for car owners even if they aren't mechanically inclined.

"If the mechanic gives you the same information, at least you know they are going down the right road," he notes. A code reader can provide car owners with one more data point to help them talk with their mechanic and avoid costly or unnecessary auto repairs. Just keep in mind that you'll have to figure out where your car's OBD-II port is (typically somewhere in the driver footwell) and get comfortable with plugging in and operating the code reader.

Possible false flags from a check engine code

With the code and its meaning in hand, a do-it-yourself interpretation can still be a little tricky — even if you are mechanically inclined, as we found out from one of our colleagues.

His wife's car started running poorly, and there was a check engine light. His code reader detected a code for the cam angle sensor. He thought about buying the sensor and installing it himself. But if he had, he would have wasted time and money because it turned out that the sensor was fine. Instead, mice had gotten under the hood and had chewed some of the wires leading to it.

Don't ignore that light

Mazor says that some people freak out when they see the check engine light. "They just put a piece of black tape over the dashboard light and keep driving," he said. But Mazor adds it's important to address problems indicated by the light promptly. Ignoring them could lead to larger, costlier problems later.

If the light comes on, Mazor suggests the driver check the gas cap. A loose gas cap sends an error message to the car's computer, reporting a leak in the vapor recovery system, which is one aspect of a car's emissions system. If the fuel cap is loose, tighten it and continue driving. Even so, it will take some time for the light to go off, he says.

What should you do if the check engine light comes on and it's steady rather than flashing? The most obvious answer is to get the engine checked by a mechanic, but you may be tempted to ignore it, especially if you know that a steady light is less of an immediate concern than a flashing one. Indeed, many people simply do nothing, perhaps fearing an expensive repair bill. Some drivers with older vehicles may want to squeeze out as many remaining miles as possible without visiting a service garage.

How to turn off the check engine light

Your vehicle is not likely to pass a state safety inspection with the check engine light on. This can be a good motivator for dealing with the problem. Ultimately, the engine and the emission control system are so interlinked that the health of the emission control system is a good indication of the general health of the car's engine. And having the issue resolved by a mechanic is the best way to make your check engine light disappear.

Still, some people might want to turn off the light temporarily, for a number of reasons. Most code readers will allow you to turn off or reset the check engine light. But this action alone does not actually address the underlying problem. In many cases, you're only delaying the inevitable and the light will simply come back on later.

Brief history of the check engine light

Automakers started standardizing their systems with 1996 model-year vehicles under a protocol called OBD-II, which instituted a list of diagnostic trouble codes and mandated that all cars provide a universal connector to access this information. The connector is usually located under the steering column and is relatively easy to access. Before 1996, carmakers had their own engine diagnostic systems, primarily to ensure their cars were compliant with EPA pollution control requirements.