Why is My Check Engine Light On?

You’re driving along, minding your own business, and the check engine light suddenly pops up on the dash. You get a lump in your throat and your heart begins to sink. 

Since the 1980s, the check engine light has been instilling fear in the hearts of motorists everywhere—and now, you’re one of them. Your mind starts to race and you ask yourself: What could be causing the light? 

Because there are a countless number of problems that can trigger the warning, the answer to that question is rarely straightforward. 

What Does the Check Engine Light Mean?

You might think that the check engine light (CEL) means that there’s a problem with your engine. And it can mean that, but it can also indicate a myriad of other issues.

Basically, the check engine light comes on whenever there’s a problem that could increase vehicle tailpipe emissions. That means anything from a slipping transmission to a bad catalytic converter could be suspect. In 1988, the California Air Resources Board (CARB) issued legislation prompting automakers to implement the light in this way. 

Here’s how it works: 

A computer, which is often referred to as the powertrain control module (PCM), monitors select systems throughout your vehicle for faults that could increase emissions. When the PCM detects a  problem, it turns on the check engine light and stores a diagnostic trouble code (DTC) in its memory.

A professional technician (or a skilled DIYer) can retrieve the code using a scan tool or code reader. The code can then be used to help troubleshoot the vehicle. 

Reasons for the Check Engine Light 

Unfortunately, there isn’t a one-size-fits-all answer to why your check engine light is on. As was mentioned, anything that could lead to an increase in tailpipe emissions has the potential to turn on the light. 

Some of the more common check engine light triggers include: 

Sensor Problems 

Your car has dozens of sensors dedicated to the powertrain (engine and transmission). If any of these sensors fail, the check engine light will appear on the dash. 

Circuit Problems

These days, even entry-level cars contain miles and miles of wiring. Almost any part of that circuitry (i.e., wires, fuses, etc.) that pertains to the powertrain can illuminate the check engine light.

Module and Data Network Problems

Modern cars also contain dozens of computers (aka modules) running tens of thousands of lines of code. Those modules communicate with one another over a data network. A problem with any of the powertrain-related devices or lines of communication can cause the check engine light to turn on. 

Ignition System Problems

Your car’s engine needs three primary things to run (and run well): 

• precise air/fuel mixture
• adequate compression
• proper spark 

The ignition system provides the spark portion of that equation. A failure in the ignition system can easily cause a misfire, which causes an increase in tailpipe emissions. The end result is an illuminated check engine light.  

Air/Fuel Mixture Problems

The engine in your car needs a precise mixture of air and fuel to run properly. When that mixture is thrown off, tailpipe emissions will increase and the check engine light will pop on. 

Engine Mechanical Problems

Remember how we mentioned that your car needs three things to run properly, one of which is compression? Well, engine mechanical problems can cause a loss of compression, resulting in an engine misfire and increased emissions. Then, our friend, the check engine light, turns on. 

Variable Valve Timing (VVT) System Problems

Most modern engines have a variable valve timing (VVT) system. The technology allows for adjustment of the camshaft angle, thereby altering when the engine’s valves open and close. A collection of oil control solenoids, camshaft actuators (phasers), and related components make this magic happen. 

Should any of those parts fail, the check engine light will illuminate. 

Emissions Equipment Problems 

A typical modern car contains all kinds of emissions equipment, ranging from a catalytic converter to an exhaust gas recirculation (EGR) valve. As you’d expect, a problem within one of these systems can turn on the check engine light. 

Automatic Transmission Problems

Because your car’s transmission directly affects engine operation and visa versa, a problem with the transmission can lead to an increase in tailpipe emissions. A control module monitors automatic transmission operation and turns on the check engine light if it detects a problem. 

What to Do When the Check Engine Light Comes On

You should address an illuminated check engine light as soon as possible. In some cases, the light can suggest a problem that’s serious enough to cause additional damage to your car. The warning also indicates that your vehicle is emitting a greater level of tailpipe emissions—and that means you’ll fail a state emissions test. 

Checking for diagnostic trouble codes (DTCs) is the first step towards resolving a check engine light concern. Although DTCs don’t tell you exactly what’s wrong with the vehicle, they do serve as a starting point for further troubleshooting. 

Codes can be retrieved using a scan tool or code reader. You can find more information about these codes on the In the Garage blog.

Once you (or your mechanic) have figured out what’s triggering the check engine light, you can move forward with the necessary repairs, then clear the codes. 

After the vehicle is diagnosed and repaired properly, the check engine light should go off and stay off.