Why Is My Check Engine Light On?

Why Is My Check Engine Light On?
May 25, 2021
Why Is My Check Engine Light On?

The check engine light on your car is a useful alert system that can mean a very simple problem like a loose gas cap or failed sensor, or it can indicate a serious engine problem like a failing catalytic converter. Different vehicles have different versions of this light. On some vehicles, it's called a check engine light, but on other vehicles, it's called a service engine soon light. Regardless of how it appears on your vehicle, a service engine soon indicator has the same function--it lets you know that something is wrong with your car.

Why Is My Check Engine Light On?

If your service engine light comes on, the first thing you need to know is why. All vehicles made after 1996 in the United States have an OBD II port that can be used to diagnose a check engine light. For a long time, special diagnostic tools like an OBDII scanner were used to do this. The tools were generally very expensive, so most people would take their cars to a mechanic for a diagnostic. These days, you can get inexpensive Bluetooth OBD II adapters that connect to your cell phone. If you go this route, there are apps available to retrieve diagnostic trouble codes to find out what's wrong with your car.


If you don't have a code reader, then you can consult this list on common reasons why your check engine light may be on, but you may ultimately still need to take it to a mechanic or get a code reader to clear the light and/or find the exact problem.

  1. Oxygen sensor failure. This is one of the most common reasons for a check engine light. These sensors frequently fail every 80,000 miles or so. Sensor failure might seem benign, but if you don't replace them, you'll eventually experience poor fuel economy and possibly damage to your spark plugs or catalytic converter.
  2. Loose, damaged, or missing gas cap. This is another common reason and is an easy fix. A missing or bad gas cap makes it difficult for your fuel system to function properly. You might lose fuel due to evaporation, and your recirculation system won't work properly. The good news is that it's an easy fix. Simply check your gas cap, and if it's damaged or missing, buy a new one. Testing if your fuel cap is bad can be done easily with the proper fuel diagnostic tools.
  3. Bad catalytic converter. This is a pretty serious problem, and it's important to note that cats don't typically fail suddenly on their own. Something else wrong in the engine causes them to fail or not work properly. This issue should usually be diagnosed by a mechanic.
  4. Mass airflow sensor. Like the other problems, a bad or dirty MAF will result in poor fuel economy and may cause engine damage eventually. You can sometimes clean the sensor to fix any problems it's having, but it might need to be replaced.
  5. Misfires. This is not a part but rather, an engine behavior. It could be due to several things, including faulty fuel injectors, defective ignition coil, worn spark plugs or spark plug wires, or a vacuum leak among other issues. It's important to diagnose and fix whatever's causing a misfire as it can result in engine damage.

What Does Service Engine Soon Mean?

Some vehicles use this terminology instead of check engine. If the light comes on, it means that the vehicle's computer has detected a problem. If the light stays solid, you're usually safe to keep driving if you don't notice any obvious problems. Sometimes the light will go out by itself as you drive. However, if the light goes out, that doesn't mean the problem is solved. It's likely to come on again in a similar circumstance. If your service engine light is flashing, then that usually means a serious problem, and you should pull over and have the vehicle towed.

How To Use An OBD II Adapter

If you decide to take the route of using an adapter to diagnose your car, here's a quick guide on how to get set up. First, locate your OBD port. Legally, it must be within two feet of your steering wheel, but this can still be in different places on different vehicles. Check near your fuse panel or underneath the wheel. Plug your adapter in, then pair it to your phone using Bluetooth. The next step is to open whichever OBD II app you've decided to install and connect it to your car. Follow the on-screen instructions to get trouble codes for your car. It's important to remember that trouble codes aren't always foolproof. Some of them can indicate multiple problems. If in doubt, you should still consider taking your car to a mechanic.

Resetting The Check Engine Light

After you've fixed a check engine light, the next issue is to get the light to turn off. Often the easiest way is simply to drive until it turns off. When you next turn on your car and start driving, the computer will recheck the problem that led to the light turning on. If the issue is truly fixed, the light should go off on its own. If it doesn't, you may need to recheck the issue. Some vehicles require you to turn the car to the On position, leave it for a second, then turn it off. Repeat this three times.


Another potential fix is to disconnect and reconnect the battery to reset the computer. To do this, you'll need to disconnect the positive power cable from the battery. Wait 15 minutes, then turn the key in the ignition to the On position, leave it for a second, then turn it off. Repeat this three times. Reconnect the positive cable and start the car.


The final solution is if you've used an OBD reader. Sometimes these readers are capable of clearing the codes as well as diagnosing them. Follow your reader's instructions to do this.


PartsHawk has you covered with a wide selection of replacement and performance car parts. Whatever your diagnostic turns up, rely on us for the replacement parts. PartsHawk is one of the most trusted names in online car parts sales.

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