Why Is The Check Engine Light On?

Few things in anyone's car ownership experience would make your heart sink any deeper than seeing the dreaded 'check engine light' come flashing on. That absolutely gutted feeling is likely because every time that light blinks, it highlights very big problems within your car that will almost always require equally big repairs and pricey bills to follow.

But this is not a warning sign that you can plead ignorance with. Leave it blinking for long enough without a check-up, and your car might suffer from catastrophic engine failure. Simply put, ignore the check engine light, and the issues will only get worse. But could they really be that bad? Well, funnily enough, it can come on even for faults as simple as your fuel cap is a tad loose.

In that sense, the check engine light is quite misunderstood, as it could be trying to alert on a wide variety of differing problems. Some of them are major, while others are trivial. Nevertheless, seeing it turn on is a sign that you should get your car checked, just to make sure. We can, at least, narrow things down to a few key reasons why your check engine light is on in the first place.

1. Loose Fuel Cap

Starting with the most simple, and harmless, we have the aforementioned loose fuel cap. You might think the cap is only there to keep the fuel in, and you would be mostly right. However, it also serves to prevent fumes from leaving the fuel tank, thus affecting the pressure of the fuel within. The result of which can have an impact on the efficiency of your car's fuel delivery system. 

This may even affect how your car drives if the sensors are detecting that something is amiss. A loose fuel cap is one of the most common triggers of a check engine light. Should it come on immediately after a fuel stop, then you may want to tighten the fuel cap, or make sure you've not left it behind. Otherwise, you may need to replace it with a new fuel cap.

2. Oxygen Sensor Failure

Every modern car has something called an O2 sensor, or oxygen sensor. Its primary role is to check and measure the amount of unburnt fuel inside of a car's exhaust system. Based on the data of how much O2 is left behind, it can then send it to your car's central computer brain, which will then be able to coordinate and regulate more effectively the mixture of air and fuel to enter the cylinders.

In essence, the O2 sensor will calibrate the most efficient combustion process as is possible. That said, O2 sensors can fail over time. When it does happen, your car won't break entirely. However, it will start burning more fuel than is necessary, which can wear out or damage other parts such as the catalytic converters or spark plugs. Some modern cars have more than one O2 sensor.

3. Catalytic Converter Failure

Speaking of catalytic converters, emissions-related problems will trigger your check engine light to flash. The catalytic converter is a part of your car's exhaust system. Its job is to scrub away the dirty carbon monoxide released during the combustion process of the engine and turns it into more tolerable carbon dioxide. Although designed to last a long time and is durable, they can fail.

When it does, it will prompt the check engine light to appear. Catalytic converters are unfortunately among the most expensive repairs and replacements that you can do on a car. Thus, it's worthwhile to keep it in good condition before it wears out and breaks completely. Something as simple as keeping up your car's regular maintenance can prevent excess strain to the catalytic converter.

4. Mass Airflow Sensor Failure

The MAF, or mass airflow sensor is one of the most important units in your car's combustion cycle. Working on the opposite end of the O2 sensor from earlier, the MAF sensor monitors how much air is entering your vehicle. This data, once again, will be communicated to your car's central computer brain, where it can then calculate the most ideal mixture of fuel and air to match.

The use of a MAF sensor, therefore, can adjust how your car is burning fuel depending on say, if there is a change in altitude or climate. Without the MAF sensor, your engine will run inefficiently and could start wearing down key components. When it does fail, it can either inform you with a check engine light, or through signs like a rough idle, reduced fuel mileage, or trouble starting.

5. Spark Plug Or Ignition Coil Problems

Aside from emissions-related faults, defects within the combustion system of your car will also cause the check engine light to come on. One of these examples includes a problem with either the spark plugs or the ignition coil. They function as one, where the ignition coil would generate and provide electricity that the spark plugs would require to spark the air-fuel mixture to combust.

Most modern cars have one coil per cylinder, so if you have a V6, it would have six ignition coils. As with most things on a car, they, and the spark plugs, will require a replacement after some time. They can cause inconveniences over time such as developing a misfire, or if there is a sudden loss in performance. If they have an issue within them, it will let you know through a check engine light.

6. Vacuum Leak

Many cars today have a vacuum system installed that's delegated to handle a lot of things at once. For example, the brake booster is vacuum-operated, while the vacuum system as a whole helps to reduce your car's toxic emissions by routing the fumes as the fuel evaporates through the engine. If the check engine light comes on, though, it might signal to you that there's a vacuum leak.

This may or may not occur at the same time you're experiencing issues like your car idling at very high and unusual RPMs. Vacuum leaks are caused by numerous things, such as the vacuum hoses themselves drying out and crack as it's exposed to constant temperature changes. Meanwhile, it may also be caused by cracked fittings or loose connections.

7. Exhaust Gas Recirculation Valve Failure

This more or less functions like the vacuum system, in that the exhaust gas recirculation valve, or EGR valve, redirects gasses from one end of the car to the next. In this case, it can lower the amount of nitrogen oxide from a car's engine, and thus reduce the emissions and makes it run more efficiently. It can direct the hot exhaust gasses back into the engine's combustion chambers.

This warms up the fuel, and makes it easier to burn. However, the EGR valve can get clogged up after a while, and would need a service. Otherwise, they could fail completely. In either case, it will prompt you to have it checked out through a check engine light. If not, you can expect to see your car releasing more emissions, losing performance, and burning more fuel.