The engine warning light truly is one of the greatest enigmas of the modern automobile. On the one hand, it is one of the most important indicators on your dash display. It does, after all, alert you early to issues and problems that are affecting one of the most important components of the vehicle, the engine itself.
On the other hand it could be a bit more descriptive, right? Sometimes it can be difficult to work out just what the light is trying to alert you to. That can lead to expensive trips to the mechanic or, even worse, the light being ignored completely. In this article we’re going to look at 5 of the most common reasons why that warning light may be lit. We’ll look at the common symptoms of each one so the next time that light goes on, you’ll know exactly what it is warning you of and, more importantly, how you can repair it.
What is The Engine Light?
When one of the Car Bibles team was at college, he went through a brief phase of wearing ripped t-shirts and listening to Green Day. People knew who he was, but to him they didn’t know who he was.
The engine warning light is far less obnoxious but otherwise quite similar. We all know what it looks like and we all know where to find it on the dash display – but what really is it?
The check engine light is just the visual indicator of the onboard diagnostic system, otherwise known as the OBD. On modern cars onboard computers are used to control just about every function of the vehicle. From the correct fuel mix to RPM and even telling the automatic transmission when to shift, the OBD pulls the strings that make the car work.
When something happens that the onboard computers don’t like or don’t understand, the check engine light is lit to tell you the car computers aren’t happy with some element of the engine and fuel systems. In modern cars, it should also be possible to attach a scanner to the car computer that can often give you a better idea of why the engine light has activated. Check out you car’s user or owner manuals for more information on this.
Now we know what the light actually is, lets look at the 5 most common things that can light the lamp.
Symptom 1. Loose Fuel Cap
Ah, the beloved loose fuel cap. This is our favorite symptom of a lit check engine light for two very good reasons. One, it is one of the most frequent causes of a warning light there is and two it is very easy to repair.
In fact as a general rule of thumb, unless your car is displaying one or more of the other symptoms we will talk about below then there is a good chance the problem could be a faulty fuel cap.
Why is It Lighting The Lamp?
The fuel system in a vehicle is of course quite the complex marvel of modern engineering. One part with a very important role that is often overlooked however is the fuel cap. Have you ever noticed that when you open the cap to fuel the car, you can often hear a soft hiss? That is escaping air, as the fuel cap helps to keep the right pressure in the tank, allowing fuel to flow through the system properly.
How To Repair It
This is a very complex repair that is going to require every tool you have at your disposal. Are you ready? Right, if the light comes on and there are no other symptoms like excess revving or a juddering when puling away, then in that case you need to go and make sure your fuel cap has been tightened properly.
That’s it, just screw it closed. It’s also a good idea to look for cracking around the cap and the seal too, and if you notice any then just buy a replacement cap for a few bucks.
Job done.
Symptom 2. Dirty Oxygen Sensor
Down at the hot end of the engine system, usually near the rear of the exhaust pipe, you will find the good old catalytic converter. The catalytic converter is one of the hardest working parts of the car. It works tirelessly to scrub un-burnt fuel and other nasty chemicals from the exhaust gas.
We’ll actually talk a little more about the converter later in this article, but for now we’re looking at one of the oxygen sensors attached to the converter itself. To make sure that the converter is working properly, an oxygen sensor is placed at the exit of the converter. This sensor works to measure the amount of oxygen coming out of the system. The cars onboard computers take this information and can calculate how much fuel the engine is not burning.
Why is It Lighting The Lamp?
That is useful to know because it allows the computer to monitor engine performance and adjust the mix of fuel and air entering the engine. It is less useful when the sensor becomes jammed with fuel and ash molecules to the extent that it is no longer able to provide an accurate reading. This can result in a lower MPG as the engine performance suffers.
How To Repair It
The easiest repair is to flush the system with a high quality catalytic converter cleaner that is formulated to also clean the oxygen sensors. Falling that, you will need to replace the failed sensors. Your car manual should highlight their location on the vehicle, and as they are externally mounted to the exhaust system they are actually quite easy to replace.
Symptom 3. Ignition System Fault
The ignition system, it goes without saying, is an integral part of the vehicle, but again its complexity can be underestimated. These days we’re used to just turning a key – or even hitting a little button – and the car roars into life. If that roar is more like a splutter these days though, then it could be a fault with the ignition system that is causing the problem and lighting up your check engine sign.
Why is It Lighting The Lamp?
Once you hit the ignition switch or turn the key, the ignition system is activated. It sprays a vapor of fuel and air into the engine cylinders which is then ignited by a spark plug (in a gasoline powered car) or a by the combination of cylinder pressure and a glow plug (in a diesel car or truck).
In a normal, gas powered car it actually takes a lot of voltage to get that spark, well, sparking at the correct level to ignite the fuel and air mix. This then can place a lot of strain on the ignition system itself, and there are two components that can, and do, fail. The first are the spark plugs and the second are the cylinder coils.
How To Repair It
The spark plug repair is one of the classic car fixes, a nice and easy job for a lazy Sunday afternoon. Simply remove and clean each plug before replacing them into the ignition system. Spark plugs do have a limited life span, so if you replace them and see no improvement (and the check engine light is still on) you may need to invest in a new set of plugs.
If the issue is with your cylinder coils then this is a more complex repair. Luckily we’ve written about how to repair and replace your coils already, in this article right here. Aren’t we great, huh?
Symptom 4. Worn Out Mass Airflow Sensor
The mass airflow sensor is quite similar to the oxygen sensor we talked about above. It is another sensor that monitors airflow in the engine to allow the car’s onboard computers to adjust the fuel mix. Unlike the oxygen sensor, which is right at the tail end of the process, stuck away near the end of the exhaust system, the mass airflow sensor is front mounted.
It is stuck right on the front of the engine, just behind the air filter where it measures air going into the engine.
Why is It Lighting The Lamp?
Unlike the other components on this list, the mass airflow sensor will often light up the check engine light on behalf of a different component. The engine air filter, which is almost always mounted in front of the mass airflow sensor, has a very dirty job. It filters inbound air before it hits the engine meaning that the air the engine mixes with fuel to burn is as clean as possible.
Unfortunately the air filter itself has no sensor attached, so the car can’t tell you when it is too dirty to operate properly. What it will do is to tell you that mass airflow sensor is not working.
How To Repair It
As we mentioned above, the airflow sensor often gets the blame for the failings of the air filter. If you are seeing a big reduction in your normal MPG and are perhaps also experiencing misfires in the engine then the check engine light could be pointing toward your airflow sensor.
Instead we suggest you take a look at the air filter. Hold it up to a light source like a bulb and if no light can penetrate the filter then you will need to pick up a new one. You should aim to replace your air filter at least once a year, and doing this will help to protect the mass airflow sensor.
If the sensor itself has become gummed up with dirt and crud, it is the same process as we talked about earlier with the oxygen sensor – simply remove it and replace with a new one.
Symptom 5. A Damaged Catalytic Converter
Finally, we’re back at the good old Cat Converter. As we talked about above, this hard working component works to scrub carbon monoxide, fuel that has not burnt and other bad things out of the exhaust gases. It is full of precious metals that act as catalysts. They convert these harmful chemicals into less harmful ones such as carbon dioxide and hydrogen (water vapor).
Why is It Lighting The Lamp?
If the converter is the issue then in addition to the check engine light you may notice a real reduction in the Miles per Gallon your vehicle can attain. You will also notice a drop in power – sometime you will really need to stand on the accelerator pedal just to get the vehicle moving.
Remember that the cat converter is only one part of the exhaust system. Sometimes issues with the converter can present as problems with the oxygen sensors (see above for more on that). As we’ll see below, replacing the cat converter is more expensive – in terms of time and money – than the oxygen sensors. Unless you are sure it is the converter, for example if you have a positive reading from a sensor plugged into the vehicle computer, it can be a good idea to investigate the oxygen sensors first.
How To Repair It
Unfortunately there is no repair. A catalytic converter does have a long but still limited lifespan and will need replacing over time. Because of the precious metals held in the converter, the unit itself can set you back hundreds of bucks. Add in labor charges of a mechanic and you could be looking at a $1,000 to $2,000 bill. It should be noted though that it is possible to replace the converter yourself and save some cash.
We also recommend a regular clean of the converter with a high quality fluid, which can really help to extend its lifespan.
Conclusion
That’s about it folks, the five most common symptoms of a lit check engine warning light. As we pointed out above, the light itself is connected to the OBD onboard computers. It is therefore usually going to light up because a sensor is not able to provide the reading the computer needs.
The above list gives you a good idea what to do when the light has already come on. But a good regime of car maintenance, including regularly cleaning all the components and sensors we talked about above, can go a long way to keeping that light switched off, and your car running like a dream.
Sources:
- What Does the Check Engine Light Usually Mean? – howstuffworks
- How to Reset a Check Engine Light – wikiHow
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