It is a bright, beautiful Monday morning.
You are heading off to work to tackle a few more tasks to get ahead. It is going to be a busy week. Suddenly, from out of nowhere, that pesky little “check engine” light decides it wants to decorate your dash.
Instead of getting ahead at work, you find yourself getting further behind as you sit at the shop and wait for your car to be repaired. Imagine your surprise when you are told that the reason for the check engine light (and your disrupted day) is nothing other than a loose gas cap!
“My gas cap was loose? What does my gas cap have to do with it? I thought the light said ‘check engine’. Was I mistaken?”
A loose gas cap is not the only reason a check engine light can come on. There are hundreds, if not thousands, of reasons this could happen. Covering all the individual reasons the light comes on could take volumes, so we will look at a general overview of the reasons behind this little light.
Automotive manufacturers are at the mercy of your favorite uncle (Uncle Sam, if you have forgotten) when developing vehicles. You are probably familiar with the Environmental Protection Agency, but you may not have realized how this organization affects the vehicles we drive.
To begin, an engine inhales fuel and air, then burns it to produce the power we need to move down the highway. It is very important that the proper amount of fuel be mixed with the precise amount of air so that you get the best possible gas mileage, and the EPA gets the least possible harmful emissions. It’s a win-win situation.
Using a little information that we learned in basic Chemistry, we know that the air is approximately 21% oxygen and 78% nitrogen. We combine air (oxygen and nitrogen) with fuel to make our motor run. Fuel is a hydrocarbon. The basic chemical process that is taking place in your engine is this:
N + O + HC à N + H2O + CO2
In short, mixing the air around us with fuel (HC) we end up burning that mixture and producing nitrogen, water, and carbon dioxide.
There’s no problem with water, right? After the summer we’ve had, I am sure that we would all welcome any water we can get! Likewise, there is nothing wrong with carbon dioxide. Our trees take it in and put out oxygen in the process of photosynthesis, and we all know oxygen is a good thing. As far as the nitrogen, we had that in the first place! So, it looks as if what is coming out of the exhaust pipes of our automobiles is perfectly safe, right?
Wrong. These are the outputs only when the air and fuel that enter the engine is completely consumed. We call this “complete combustion”, and, in reality, it does not occur. There are always leftovers.
These leftovers are in three forms: Hydrocarbons (unburned fuel), carbon monoxide (partially burned fuel), and nitrogen oxides, which result when temperatures become high enough for the oxygen and nitrogen in the air to combine.
So, now that we have an understanding of the harmful pollutants that are leaving our cars, let’s go back and talk about our favorite uncle. He wants a clean environment for everyone, and the best way he can ensure that is by being certain automobile emissions are as harmless as possible. Therefore, the EPA requires auto manufacturers to turn on a check engine light whenever emission levels rise beyond a set level.
Now, think back to that gas cap we left loose. What happened? As far as your car is concerned, there was a hydrocarbon emission. The fuel was evaporating from your car’s gas tank the same way it would if the top was left off a fuel can in a garage. Auto manufacturers are required by the EPA to keep a watch on this. Watching with its electronic eyes, the car knows that if fuel is escaping from the gas cap, then it better turn on the check engine light.
The next time you go to fill up, here’s a tip: once the gas cap is tight and begins clicking, turn it one more revolution. This will ensure your gas cap is tight enough, and may save you an unwanted delay in your workday.
Hunter Taylor is the automotive department head at Halifax Community College. If you have any questions about this article or other aspects of automotive technology, please contact Hunter at