
You know it, you’ve seen it, and you probably hate it. The ever pesky check engine light. It’s nice that car’s these days have computers that can tell you when something is going wrong, however it totally sucks that sometimes the car’s dash indicator lights can be so enigmatic, the CEL being no exception to this rule.
Most drivers dread the check engine light because it can be such a hassle. You need a special handheld computer in order to find the error code that will actually tell you what’s wrong, and even once you’ve obtained that diagnostic code the answer might not be entirely clear. Many times the check engine like coming on is no big deal, it may just represent a minor issue with a cheap fix. Every so often when that check engine light comes on it could cost you thousands of dollars.
Being that the check engine light can be the bane of some drivers who are not so mechanically inclined, and can sometimes induce pure panic, we’ve put together this guide of some of the most common reasons your check engine light might illuminate to ease your fears.
Faulty O2 Sensor

Failure of one or more of your vehicles oxygen sensors is one of the most common things to trigger the check engine light. This small sensor monitors the oxygen level in the combustion chamber of your engine and is important to ensuring the right oxygen and fuel mixture for optimal fuel economy. If you have a car older than 1980 you probably won’t have to worry about this one (then again you may not even have a check engine light at all).
Like all of the components on your vehicle, the O2 sensor can get dirty over time. When the oxygen sensors under the hood get gunked up they will be unable to measure the oxygen level in the fuel mixture properly, triggering the check engine light. Unfortunately these aren’t always inexpensive to replace. Depending on the make and model of your vehicle replacing an O2 sensor could end up costing you a couple hundred bucks.
You Just Need To Screw On Your Gas Cap!

It may sound ridiculous, but failure to screw on your gas cap tight enough can actually trip the check engine light. The fuel vapors escaping from the gas tank reduces the level of the vacuum pressure in the engine, throwing everything out of whack and triggering the CEL.
Fortunately, this one’s a cheap and easy fix. If you notice your check engine light coming on shortly after fuelling up, a loose gas cap is likely the culprit. All you need to do is turn your gas cap a couple more turns until a few audible clicks. The check engine light may not turn off until after you’ve driven a few miles, but it should resolve itself soon.
Bad Spark Plugs

Spark plugs are simple devices, but play a huge role in the proper operation of your engine. They provide the first little spark that ignites the fuel combustion process that powers your engine. If your spark plugs are dirty or otherwise not working properly, they can trigger the check engine light. One of the symptoms of a bad spark plug is jerky acceleration.
If your spark plugs are causing the check engine light to come on, there’s no fear. They are cheap and easy to replace. Usually they are easily accessed and can be changed with just one wrench. For this reason, it’s a good idea to change your spark plugs regularly so that they never get the chance to start causing your real problems.
Bad Catalytic Converter

The catalytic converter sits under your car behind the exhaust manifold. It uses precious metals like platinum, palladium, and rhodium to filter the car’s exhaust of harmful pollutants like carbon monoxide and prevent them from entering the atmosphere. If you ever fail an emissions test, it’s the catalytic converter that is usually to blame.
Failure of the catalytic converter can produce an awful smell of rotten eggs. Worse, it can lead to lower fuel economy, sluggish acceleration, and can even choke out your engine altogether.
Many vehicle owners may never have to replace their catalytic converter, however if you do have to replace one, it can get spendy. Due to the materials used to manufacture catalytic converters they can end up costing several hundred dollars just for the replacement.
Faulty Ignition Coils

Ignition coils are the wires under the hood that carry the spark from the car’s battery to the spark plug. They store energy inside the coil, releasing it to the spark plug when it comes time to start up the engine.
Just like the spark plugs, the ignition coils will need to be replaced every so often (just not quite as frequently as the spark plugs). The reason is simply that they can wear out. The rubber holding the wires together can become cracked and degraded allowing the charge inside the cable to be transferred to other metal inside the car instead of the spark plug. If the spark plug isn’t receiving the energy it needs to ignite the fuel mixture, the check engine light may be triggered.
Luckily replacing your ignition coils is an easy job. Most cars have just one ignition coil per spark plug, and they are usually pretty easy to access. And, unless you drive a hot rod, a new ignition coil shouldn’t cost you more than around 20 bucks.
Bad Fuel Injectors

On top of each cylinder in your car is a little device called the fuel injector. This little valve does exactly what its name says; inject fuel into the combustion chamber. It is controlled by your car’s computer brain and works in tandem with the oxygen sensors to promote the optimum fuel and oxygen mix for peak performance. When the fuel injectors are clogged or otherwise faulty you may get an error code resulting in a check engine light.
Over time fuel the tips of the fuel injectors can become clogged by the impurities in the fuel that you are using as well as by residue left over from the combustion of that fuel. Some symptoms of bad fuel injectors include cylinders misfiring during acceleration or a rough idle. If you have a faulty fuel injector you will need to get it replaced, but it can end being costly, with some estimates coming in at over $700.
Bad Thermostat

The thermostat works to tell your car’s computer brain, as well as you – the driver, the operating temperature of the engine. This is important because the engine needs to operate within a certain zone in order to get the best fuel economy as well as reduce emissions. You as the driver also need to be aware of the engine temperature because if the engine is overheating you need to pull over right away and address the issue. If you have a faulty thermostat, it could trigger the check engine light.
Luckily the thermostat in your car is a relatively simple device, and it’s easy to replace. With a few simple tools and some rudimentary knowledge under the hood, you can easily replace it yourself. However, if you’d rather let the pro’s handle it, it shouldn’t end up costing you more than a couple hundred bucks.
Vacuum Leak
Some of the critical vehicle systems including the power brakes and steering, emissions systems and throttle require negative pressure to function properly. Vacuum pressure is produced by the intake manifold, then transferred to each of these systems through a series of valves and hoses. If any of these are compromised in any way it may be indicated by the check engine light.
In order to get to the bottom of an issue with the engine’s vacuum pressure you’re going to need a trained eye. The issue may be simple, like replacing a faulty valve or hose, or even reconnecting a vacuum hose that has come loose, or it could be more serious, like a problem with the intake manifold itself. Either way, you are going to need a skilled mechanic to take a look if you are going to get to the bottom of the issue.
Faulty Mass Airflow Sensor
Your car’s engine needs 3 things to produce power: fuel, oxygen and spark. The magic all comes down to the fuel and oxygen mix. The mass air flow sensor helps the car’s computer brain to know how much air is coming into the engine. If the mass air flow sensor is faulty, it could trip your check engine light and cost you about $200 to replace.
If your onboard diagnostic system is throwing a code indicating a problem with your mass air flow sensor, don’t go running to the shop to have it replaced right away. There is another culprit that may be causing your problem: the engine air filter.
The engine air filter is one of those items of routine maintenance that often gets overlooked. After all, out of sight out of mind, right? Most people don’t think that having a dirty air filter can really do much harm, but it can cause the mass airflow sensor to malfunction and relay faulty information to the computer. If you suspect an issue with your mass airflow sensor try replacing your engine air filter first!