When your car's check engine light illuminates your dashboard, it's usually accompanied by a sinking feeling in the pit of your stomach. 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.
The check engine light — more formally known as the malfunction indicator lamp — is a signal from the car's engine computer that something is wrong.
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.
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.


Deciphering the Code
Some drivers may 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 $75. But there's a way to preview what the problem might be. Do-it-yourselfers can buy inexpensive 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.
How to Turn Off the Check Engine Light
Most code readers will allow you to turn off or reset the check engine light. But this action alone does not actually repair the underlying problem. In many cases, the light will simply come back on later.
Mixed Signals
But even with the code and its meaning in hand, a do-it-yourself interpretation can be a little tricky — even if you are mechanically inclined, said Dan Edmunds, director of vehicle testing for Edmunds.
"My wife's car started running poorly, and there was a check engine light. My code reader detected a code for the cam angle sensor. I thought about buying the sensor and installing it myself. But if I had, I 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," said Edmunds.
What Could Cause the Check Engine Light to Come On?
CarMD, an automotive telematics company, published a list of the 10 most common check engine codes in 2018, along with their estimated cost of repair:
1. (tie) Replace ignition coil(s) and spark plug(s) 
1. (tie) Replace oxygen sensor(s) 
3. Replace catalytic converter(s) with a new OEM catalytic converter(s) 
4. Inspect for loose gas cap and tighten or replace as necessary 
5. Replace ignition coil(s) 
6. Replace evaporative emissions purge control valve 
7. Replace mass airflow sensor 
8. Replace evaporative emissions purge solenoid 
9. Replace fuel injector(s) 
10. Replace thermostat 
Occasionally, the check engine light comes on when nothing is wrong with the car, said Steve Mazor, 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.
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 should 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 many people 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. But before they can pass their state's vehicle inspection, they have to get the light turned off. And a state inspection is a good motivator for dealing with the problem.
As Dan Edmunds points out, the system is primarily designed to continuously monitor a car's emissions system over the life of the car. However, he notes, "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."
Good Backup Plan
Mazor says that even 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. Edmunds agrees, adding that a code reader provides car owners with one more data point to help them talk with their mechanic and avoid costly or unnecessary auto repairs.

Originally Published on Edmunds.com
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