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Neighborhood social groups can be so entertaining — at least for me. While it’s true no one needs a 1,200-word lecture about keeping your dogs off that one guy’s front lawn (or else you’ll get the hose again), it thrills me to read about my neighbors’ first-world problems and how monumental they seem on a laptop screen. Such is the case with this one lady and her check engine light, which seems to have lit up on her way to pick up the children from some kind of kid practice. 

According to her post, she had never had her car serviced before (red flag), so she brought it to the first auto service she saw: a tire shop. Almost $1,500 and four new, well-aligned tires later, and guess what? The check engine light was still on!

I feel bad for my neighbor. She didn’t know that the check engine light is specific to her engine and has zero to do with her wheels. Had she reached out to the group to get recommendations for a trusted auto shop first, she wouldn’t have to stare at that little yellow warning anymore. Of course, she wouldn’t have those shiny new tires, either. You won’t have to worry about befalling a similar fate, because I’m going to lay out all the information you need about your check engine light. Put away that piece of black duct tape you were just about to use to cover up that light and read on.

What Is a Check Engine Light?

The check engine light (CEL), also called the malfunction indicator light, is part of a vehicle’s onboard diagnostics system. It gives auto technicians and DIYers a simple way to monitor and analyze repair needs. When the light illuminates, it means that the diagnostics system has detected a malfunction. 

The major systems of your vehicle are the engine, transmission, cooling and lubrication, fuel system, electrical, and the chassis, which includes braking, wheels and tires, suspension, and the actual car body. The warning light isn’t a monitor for all of a car’s systems. It won’t tell you that you’re low on oil, nor will it necessarily tell you if major engine components are failing. The check engine light mostly concerns emission sensors for the engine and a few engine-related systems such as fuel and air intake. 

There are other warning lights that will let you know about a specific problem with your car, and these are all very important. They include oil pressure, coolant temperature, battery, transmission temperature, and brake system. Do not ignore these warning lights as they could mean significant damage to your vehicle, or you could end up stranded somewhere. Read your owner’s manual to find out what all of your warning lights signify and for specific instructions on how to proceed when one is lit. 

Check Engine Light Issues

When your check engine light illuminates, it likely senses an issue in one of these areas:

  • Gas cap: Fuel up lately? When the gas cap is loose (or missing), the system can sense a pressure or air leak issue. The gas cap is the first thing to check on your own. 
  • Thermostat: Your vehicle is happiest and runs best when the engine is warm but not too warm. The thermostat regulates the amount of coolant running through the system. Thermostats are simple, and they can fail, but replacing them is a pretty simple procedure.
  • Mass air-flow sensor: This senses the air-fuel mixture and is found in the air-intake duct. If the mixture is too rich, meaning burning excess fuel, or too lean because there’s too much air, your engine could sustain heavy damage. These sensors, however, could trigger a warning light if they become dirty.
  • Ignition coil: The ignition coil sends a park to each spark plug consistently and at the proper times. If the system senses too many misfires, it will flip on the check engine light.
  • Spark plugs: If a spark plug fails to spark consistently, a check engine light could result. Spark plugs are easily replaceable and should be switched out with fresh ones about every 30,000 to 60,000 miles.
  • Plug wires: Some vehicles deliver an electrical spark from the ignition coil to the spark plug via plug wires. The connections could come loose, or the wires could suffer damage to their insulation. This is an easy at-home fix.
  • Fuel injector: The injector regulates the amount of gas that is sprayed into each cylinder during intake. It can become dirty and get clogged, or it can stick open and leak fuel. 
  • Catalytic converter: This component looks like a muffler and can be found under the car. It helps the vehicle run cleaner by burning off environmentally harmful chemicals caused by burning fuel. If it isn’t working properly, a sensor will trip the check engine light. Over the last few years, catalytic converter theft has become something of an epidemic because there are precious metals in them that are worth a bit of money — platinum, rhodium, and palladium. If your catalytic converter is missing, it will trigger the warning light.
  • Oxygen sensor: Part of the exhaust system, the oxygen sensor will throw a warning light if there is too much oxygen exiting the exhaust, which would indicate a problem with the engine’s proper functioning. There are oxygen sensors before and downstream of the catalytic converter. 
  • Vacuum leak: Your engine has valves and hoses that control the vacuum pressure. If a valve is sticky or a hose is damaged or loose, it could cause your engine to run rough and trigger a warning light. 
  • Emissions control valve: The evaporative purge valve works as a filter for fuel vapor, condensing those vapors back into liquid fuel. If it isn’t working properly, the check engine light will illuminate. Plus, you won’t pass your smog check.

Keep in mind: A flashing check engine light should be addressed as soon as possible; a solid warning light means it should be checked out soon. 

How to Diagnose a Warning Light Issue

There are two ways to determine what a check engine light is trying to tell you: Go to a mechanic or check it yourself with a scan tool. An auto technician will hook up your car to a professional diagnostic tool, which will offer a lot of information about the engine’s internals and how they are working together. It will also give out specific diagnostic trouble codes, or DTCs, which will direct you specifically to the malfunction. 

The tool a mechanic uses is the same one you can purchase, albeit a lot less sophisticated than the one in a pro’s shop, called an OBD-II scan tool. It usually plugs into your vehicle under the dashboard on the driver’s left side. Once the car is running, the scanner will display standard trouble codes. You can type in your vehicle’s make and model as well as the trouble code here to discover what the problem is. Or, you know, you can just Google “diagnostic trouble codes” and go from there. Car Bibles has a step-by-step guide on how to use your new scanner.

FAQs About Check Engine Lights

Car Bibles answers all your burning questions.

Q: Why does my check engine light go off and on?

A: If the light illuminates sometimes and not others — which is different from flashing — it may signal an intermittent fault. Get the car scanned to discover the specific issue.

Q: Do I need to buy my own OBDII scan tool?

A: You don’t have to buy one, but they are inexpensive, just in case you’d rather stay home and check it yourself. If you have a good rapport with a dealership or an auto shop, there’s a good chance they won’t charge you to give you the trouble codes. Advance Auto has a curbside service to help you out. Also, the folks at AutoZone will scan your vehicle for free.

Q: I realized my gas cap was loose so I tightened it, but the check engine light is still on. How can I turn the light off?

A: If you think you’ve fixed the problem, you can use your OBD-II scan tool to clear the trouble code and reset your check engine light. Be warned, however, that just because the light is off doesn’t mean the error has been corrected. If the light comes back on, the issue may be a little more complicated.

Learn How To Use a Scanner From This Video

The folks over at parts supplier Summit Racing set up a handy video that shows you how to use your trouble code scanner as well as how to use it to clear the codes.

Share Your Tips

You know you’ve got some good advice for those facing the dreaded check engine light. Please, add it in the comments section below. Writers (and other readers) will answer questions and comments whenever they can.

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