A vehicle’s Check Engine light (or Service Engine Soon light in some vehicles) is the automotive equivalent of the Blue Screen of Death for computer users. It’s the one thing you never want to see pop up. For those who know quite a bit about cars, it can be an annoyance. For those who know little, it can be downright frightening. Everyone knows that the vehicle is powered by the engine. If it stops, so does the vehicle. If it breaks, so does the bank. That’s what can make it so scary. Oftentimes, however, a Check Engine light can be something very simple. Let’s run through a few of the top reasons for a Check Engine light so you have a better understanding of what your car may actually be trying to tell you.

1. Check Your Gas Cap

Yes, you read that the right way. A loose gas cap can trigger your Check Engine light. This is because the most common reason for a Check Engine light is from emissions system problems. Your fuel system is a pressurized system. The computer is monitoring that pressure at all times when your vehicle is running. If you leave your fuel cap off or loose (even by half a turn) it can trigger an emissions code in your vehicle’s computer. This, in turn, will cause the computer to turn on your Check Engine light. So, the first thing you should do if your light comes on is stop your vehicle somewhere safe, turn it off, and check your fuel cap. If it is loose, put it back on and restart your vehicle. The computer will read the fuel system pressure and should turn the light off automatically after a few moments. A bad fuel cap can also cause this problem. Unfortunately, the only way to know if the cap is bad is to pull the code from the computer and see if it was indeed the problem.

2. Oxygen Sensors

Your oxygen sensors are another emissions system part that will trigger your Check Engine light. Ironically, they aren’t really attached to the engine. They are screwed into your exhaust system in various places to monitor the air coming out of your engine. They tell your computer if your air/fuel mixture is running rich or lean. Most cars today have at least two sensors, one before the catalytic converter and one after (this is to monitor the air on both sides to see if the converter is also doing its job), while some vehicles have several for redundancy’s sake. Each sensor is electronically connected to your computer. If the sensor fails or if it gets a reading it’s not supposed to be getting, it will trip your Check Engine light. The only way to tell if this is the problem is to have your vehicle scanned at a repair facility.

3. Other Emissions System Parts

With today’s emissions standards, the emissions control systems on most vehicles have become very complex. They are much more than a catalytic converter and a couple sensors. They now consist of filters, canisters, exhaust gas recirculation (EGR) valves, positive crankcase ventilation (PCV) valves, smog/air pumps, bypass valves, and several sensors to monitor the entire system. Should any of these areas fail, it can trigger any of a number of various codes in your computer. These codes all trigger the Check Engine light. Once again, the only way to find this code, which will give you an idea of what part is failing on your vehicle, is to have it scanned by an automotive professional.

4. Mileage Intervals

Some vehicles are set up so that the Check Engine light comes on at certain mileage intervals. This is there as a reminder. Your vehicle has many important birthdays. These are mileage intervals where a lot of routine maintenance needs to take place all at once. This maintenance includes tune-up, all fluids changed, brake inspection, and in some cases, timing belt replacement. This doesn’t occur in all vehicles, but there are still a few left that have the “reminder” Check Engine light equipped. Typically this happens at 30,000, 60,000, or 100,000-mile intervals. So, if your vehicle’s light pops on unexpectedly, take a glance at your odometer and see what it reads. If it just rolled one of these increments, this could be the cause. The only way to have it reset is to have your local technician reset it via computer.

5. Major Problems

This is the one nobody ever wants to hear. If your light doesn’t pop up for one of the above reasons, chances are it’s something more severe. The range of issues this encompasses is too broad, and far too scary for most, to even delve into. Some of the more common reasons are: a bad Electronic Control Module (computer), broken timing belt/chain, loss of cylinder compression/misfiring, or one of the many various electrical sensors on your vehicle has just gone kaput. None of these are very pleasant, but some can be as simple as a bad spark plug or spark plug wire. Others will have you cursing your vehicle and saying words you’d probably never repeat in front of your mother.

One important thing to remember is, any time your Check Engine light comes on, it’s because your computer has stored a code that will give you an idea of what is wrong. This code will not always tell you what the problem is. There are several places, aftermarket parts stores being the biggest, that will scan your computer and tell you exactly what code is stored. The problem is, what if the code isn’t the part that failed. For example, let’s say the code reads that you have a bad oxygen sensor. What if the problem isn’t the sensor, but the wiring somewhere between the sensor and the computer? The only way to tell this is by having your vehicle checked by a trained technician. This can be expensive, depending on your vehicle’s problem, but it could end up saving you a lot of time, hassle, and money in the long run.