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What the Check Engine Light Means

Your car’s diagnostic systems, from the ABS brakes, emissions systems, and engine diagnostic controls, can all be the cause of your car warning lights. It could be a simple, harmless problem or it could be the precursor to a major meltdown. Take a look at a handful of typical warning light causes:

Oxygen Sensor

Most vehicles have at least two oxygen sensors, an upstream and a downstream. Their purpose is to measure the emissions in the exhaust, and the readings are used to adjust your engine’s performance in real-time. The most common cause of an illuminated Check Engine light according to CarMD is a faulty oxygen sensor.

If an oxygen sensor is faulty, you could see a drop in fuel efficiency or poor engine performance. It can cause your engine to run lean, which could result in permanent, debilitating engine damage over the long haul.

Catalytic Converter

Since 1981, every car manufactured for the US market has been equipped with a catalytic converter. It superheats the unburned particles that flow out your exhaust pipe. The process turns pollutant chemicals like nitrous oxides and hydrocarbons into harmless elements like carbon dioxide and water.

If the catalytic converter is on its way out, you’ll have the Check Engine light on. You might notice a very unpleasant rotten egg odor or a rattling noise underneath your vehicle also. Unfortunately, the catalytic converter is one of the more expensive repairs you may need to tackle.

Fuel Cap

Yes, even a loose fuel cap can cause the Check Engine light to come on. The fuel cap does more than just prevent fuel from spilling out. It seals the filler neck, allowing the fuel system to become pressurized. Your car’s diagnostic systems check the fuel system for leaks this way.

If your fuel cap is loose or doesn’t seal well, the Check Engine light will come on. Some vehicles have a ‘check fuel cap’ warning or a Service Engine Soon light that may come on for the same reason. Often, tightening the cap is enough, but the fuel cap might need to be replaced if the seal is broken.


A cooling system concern can also illuminate the Check Engine light or set off your Check Gauges light. In the cooling system, the most common failure is the thermostat, a bimetal component that regulates your engine temperature through coolant flow.

If the thermostat gets stuck open, the engine may never reach its optimal temperature for proper operation. If the thermostat is stuck closed, your engine can easily overheat. It’s when the temperature gauge spikes into the red zone that the Check Gauges light comes on, or an Engine Hot warning displays. You’ll need to replace the thermostat. If the engine has overheated, the head gasket and more might need to be replaced.

Ignition System

If you have your Check Engine light flashing, it’s likely due to an ignition system problem in some way. That includes spark plugs, ignition cables, or an ignition coil. These parts are responsible for igniting the fuel in each engine cylinder. When the fuel doesn’t ignite properly, or there’s no spark at all, it’s known as a misfire.

A misfire will turn on your check engine light for sure, and severe misfires will set your Check Engine light blinking. The repair isn’t usually difficult or overly expensive, but it is critical to ensuring reliable operation. If you delay misfire repairs, you can cause additional problems like a flooded engine or catalytic converter failure.

MAF Sensor

Another cause of a flashing Check Engine light can be the mass air flow sensor or MAF sensor. It meters the air that’s drawn into your engine, either in the ductwork from the air filter to the intake or in the airbox itself. It’s a delicate sensor and even minute problems can cause it to work improperly. An issue as simple as a bug stuck to the MAF sensor can make it read erratically.

The good news is that the MAF sensor can often be cleaned instead of replaced. If it must be changed, the repair is rather inexpensive. Until that happens, though, your engine will be quite unreliable. Inaccurate air metering can cause stalling, excessive fuel consumption, or a rough run.

Emissions Sensors

A whole slew of emissions sensors can cause your car warning lights to come on. The emissions system is responsible for ensuring the cleanest engine operation possible and has several aspects. There’s an exhaust gas recirculation (EGR) system, a purge control system, evaporative emissions, and others. Of those parts, the most common failures are the purge control valve and the purge solenoid.

If a purge control valve or solenoid is stuck, the Check Engine light or Service Engine Soon light will likely be on constantly. The good news is that it’s not going to cause additional damage, nor will it affect your vehicle’s operation. It’s also one of the least expensive repairs.

ABS Concerns

You may also notice other warning lights come on such as the ABS light. While you may not think much of the ABS, it’s an important part of your car’s operation. It can be an issue with a wheel speed sensor, which can affect your ABS brakes and traction control. It can also affect your stability control system. It could be a leak in the ABS hydraulic control unit which can affect your power brakes.  Or, it could be the ABS control module at fault, which influences your braking system in virtually all aspects.

The proper repair can be as simple as a wiring repair. It may require a replacement wheel hub that houses the ABS wheel sensor, or it may be the module or control unit that needs to be changed. This innocuous light can be quite expensive to fix.

How to Reset the Check Engine Light

For common Check Engine light concerns, the warning light will likely stay on for a while after you perform the repair. There are a few ways to turn the reset the Check Engine Light:

Wait it Out

Depending on the type of code set, the Check Engine light might turn itself off after a few drives. The system performs a self-test, and if it passes enough tests, it’ll turn the light off itself.

Disconnect the Battery

In some cases, you can clear your car warning lights yourself simply by disconnecting the battery for 10 to 15 minutes. It discharges the memory in the control modules, and the light turns off…sometimes.

Have the Codes Cleared

Once the repair is completed, a code reader can turn the Check Engine light off. You can buy an inexpensive one online or in parts stores and do it yourself. Some parts retailers offer rental code readers, or you can visit your repair shop to have your Check Engine light cleared.

Engine light can lead to some serious issues down the road. It's a good idea to check the current value of your car before it's too late. Broken down car is hard to sell and selling your car for parts may not be your best option. 

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