Check Engine Lights: Common Reasons Why They're On
- by Joe Weber - updated on 6/21/2024
We've all felt that sinking feeling in our gut when Check Engine Light (CEL) suddenly lights up without warning. Sometimes when this happens it's obvious from the vehicle's performance that something is wrong.
Other times, the CEL will light up even when everything seems fine. So, how do you determine why the CEL has turned on?
Today, we'll look at six of the most common reasons your check engine light may be on, along with additional symptoms to help you identify the problem.
Common Check Engine Light Causes
When your CEL lights up, it can be caused by various issues, from simple fixes to more serious repairs. Here are six of the most common causes.
1. Loose or Damaged Gas Cap
Your gas cap performs several functions. For starters, it helps maintain the correct gas tank pressure. It's also a part of your vehicle's evaporative emissions control (EVAP) system. The EVAP system prevents fuel vapors from getting into the atmosphere.
What Causes a Gas Cap to Fail?
Your vehicle's onboard computer monitors the EVAP system for leaks and is quite sensitive. If your gas cap is loose, missing or damaged, it will detect the leaking fumes and turn on your CEL. Gas caps can also wear out over time. Gaskets can dry and crack, plastic gets worn, etc. It's unlikely to have a gas cap that lasts as long as the vehicle, it will need to be replaced at some point.
What If My Vehicle Doesn't Have A Gas Cap?
It's not uncommon for new vehicles to be engineered without gas caps. This makes it quicker and easier to fill up at the pump. Just insert the nozzle into the tank and start filling. How does this work? The cap is still there, it's just on the inside now. When you insert the nozzle, that spring-loaded latch moves out of the way to fill. When you remove the nozzle, the cap snaps shut, trapping vapors inside.
Troubleshooting Tips
If your CEL comes on right after you've filled up your tank, the gas cap is likely loose. Tighten it up and see if that solves the problem. Additionally, if you notice that your gas cap is damaged, be sure to have it replaced.
2. Your Oil is Too Low or Too Dirty
Oil lubricates the components of your engine. This reduces the friction and wear between these components, allowing them to run smoothly and helping to protect them from becoming damaged. It also helps minimize heat between the moving parts and keeps engine components clean.
Can Low Oil Cause the Check Engine Light to Come On?
Oh, you bet. Changing your engine's oil at the recommended intervals is very important for the health of the engine. Not enough oil will almost always cause low oil pressure which can cause your CEL to come on. Not changing the oil regularly can also cause the oil to become dirty, which can cause unnecessary wear on other parts of the engine.
Troubleshooting Tips
Check the oil level in your vehicle. If it's too low, add additional oil. If the oil appears dark black, have the oil changed as soon as possible. In either of these scenarios, you should bring your vehicle to a mechanic for a once-over. It's not normal unless you are not changing your oil enough, to see dirty oil or low oil levels.
3. Faulty Oxygen Sensor
The oxygen sensor, or O2 sensor or lambda sensor depending on who you talk to, is located in your vehicle's exhaust system. It measures the amount of oxygen present in the exhaust gasses that leave your engine. This data is then sent to your vehicle's onboard computer to determine the proper air-to-fuel ratio for your engine. An oxygen sensor will trigger the check engine light when your vehicle's emissions are too excessive or the sensor isn't reading properly.
What Causes an Oxygen Sensor to Fail?
Oxygen sensors will fail for several reasons, age being one of the most common. Failure can occur when contaminants such as silicates and phosphorus enter the exhaust or by using low-quality fuel or fuel without ethanol (a cleaning agent). Without a functioning oxygen sensor, your engine's fuel ratio can be thrown off causing a noticeable drop in fuel efficiency.
Additional Signs That Your Oxygen Sensor is Failing
Some other common symptoms of a faulty O2 sensor are
- Sulfur or 'rotten egg' smell coming from the exhaust
- Black smoke
- Failed emissions
- The engine hesitates, bucks or has power surges
4. Faulty Catalytic Converter
Your vehicle's catalytic filters out pollutants like carbon monoxide, sulfur dioxide, lead, nitrogen dioxide and more to provide cleaner exhaust emissions. Most vehicles have one catalytic converter but some have multiple. Catalytic converters are designed to last the full life of your vehicle but they too can sometimes fail early.
What Causes a Catalytic Converter to Fail?
Your catalytic converter can fail for many reasons
- Improper fuel mixture
- Incorrect timing
- Bad spark plugs
- A malfunctioning oxygen sensor
- Sticking float
- Defective fuel injector
- A faulty check valve
Additional Signs That Your Catalytic Converter is Failing
Faulty catalytic converters will present in many ways
- You hear rattles from or near the engine
- The check engine light is on
- Acceleration is sluggish
- Gas mileage has been reduced
- Much like the O2 sensor, you may smell rotten eggs
- Your engine misfires
5. Bad Spark Plugs
Your spark plugs create the spark needed to ignite the fuel in the engine to start. Spark plugs do more than that though, they provide the spark needed to keep the engine running and accelerate.
What Causes Spark Plugs to Fail?
For the most part spark plugs will wear out naturally with use due to the violent nature of the combustion that is present during their operation. Contamination of oil, fuel or carbon can also form on spark plugs due to their interaction with the air-fuel mix.
Additional Signs That Your Spark Plugs Are Failing
If a spark plug is bad, you may notice several issues with the engine
- You'll notice difficulties starting the engine.
- The engine may hesitate during acceleration or misfire while idling.
- You may also experience reduced gas mileage.
6. Faulty Air Flow Sensor
A mass air flow (MAF) sensor is part of the vehicle's electronic fuel injection system and helps calculate the amount of air entering the engine to allow the vehicle's computer to calculate how much fuel is needed for proper combustion.
What Causes an Air Flow Sensor to Fail?
Mass air flow sensors can become contaminated or damaged. This can be caused by a dirty or clogged air filter and not necessarily a problem with the sensor itself. Contamination from excessive oil mist from the crankcase can also cause the sensor to fail prematurely. When the air flow sensor is faulty, it won't measure the correct amount of air flow and your engine will have a fuel mixture that is either too high or too low in oxygen.
Additional Signs That Your Air Flow Sensor is Failing
Some common symptoms of a failing air flow sensor include
- The vehicle becomes difficult to start.
- You notice a worsening fuel economy.
- The vehicle has a rough idle.
- The engine stalls, jerks or hesitates during acceleration.
- You may also notice black exhaust.
How Can You Read Car Error Codes?
Reading error codes is only one side of the coin. There are countless OBD-II readers available for purchase at almost every auto parts store. But, what do the codes mean? That is where expert knowledge of a mechanic comes into play.
Your vehicle's check engine light illuminates so you plug in a scanner to see what the cause is. It displays a code for a mass air flow sensor issue. Is it the sensor itself? Is it a filter issue? Is it worse and there is a problem deep inside the engine? Only a trained mechanic will be able to figure it out.
When your check engine light illuminates, the best course of action is to take it somewhere where they can scan it for trouble codes and diagnose what's really going on.
Batteries Plus Has Your Automotive Needs Covered
Batteries Plus is here to help with your automotive battery and lighting needs. Browse our selection of auto essentials online or stop into one of the 700+ stores around the country to see what we have to offer.
Batteries Plus is proud to carry top-quality batteries for your car or truck, as well as battery chargers to help you maintain them. Plus, don't forget about our car battery installation (available on most makes and models at most locations) and convenient free battery testing. Plus, many of our stores also offer key cutting and key fob replacement. Check with your nearest store for availability.
Looking for more helpful automotive information? Read our blogs entitled "6 Reasons Your Car Battery isn't Working" and "6 Essential Car Maintenance Checks and How Often You Should Perform Them."