Top 7 Reasons Your Check Engine Light is On

While it’s often assumed that the check engine light (CEL) is an indication of a major mechanical issue, this isn’t always the case. In fact, according to many professional mechanics, a warning light can be caused by a minor electrical glitch. Still, anytime a CEL illuminates on your dashboard, it is something that should be taken seriously. A check engine light is triggered when your vehicle's onboard computer or ECU detects an error coming from one of the multiple sensors on your car, truck, or SUV. When this occurs, the ECU creates and stores an OBD-II diagnostic trouble code, which then illuminates the CEL.

When you take your car to a mechanic, they will download these OBD-II codes to give them a starting point to diagnose the root issue and complete the right repairs. Now that we’ve explained the why — let’s dig into the reasons the CEL can light-up, and the best ways to resolve these issues.

1. One Sensor has a Loose Electrical Connection or is Damaged

Believe it or not, the most common reason the CEL will light-up is due to a poor electrical connection from the ECU to the sensor. When the electrical harness is loose, frayed, damaged, or exposed, it can send a signal to the ECU and mimic a mechanical problem. It’s also common for sensors to wear out over an extended period of time. This tends to happen quicker with fuel and exhaust system sensors, which are exposed to hot gases and carbon deposits.

What can be done to fix this? In most cases, a professional mechanic will simply replace the sensor and the electrical harness to resolve this type of problem.

2. Loose or Damaged Fuel Cap

Today’s modern cars, trucks and SUV’s are very carefully monitored for fuel economy and emissions. If the gas cap is either left off, damaged, or does not maintain a solid seal, it will create less pressure inside the fuel cell. This tells the sensor there is a damaged or loose gas cap and can cause acceleration problems and/or poor gas mileage. Additional signs of a broken fuel cap include:

  • Cap does not tighten or lock properly.
  • Car smells like fuel

What can be done to fix this problem? Most of the time, replacing the gas cap can solve the problem. However, you’ll have to contact a professional mechanic to complete a check engine light inspection in order to remove the code and reset the warning light.

3. Damaged O2 sensor

The emissions system on your vehicle is highly complex. It starts with a series of sensors that monitor the fuel/air ratio and the amount of carbon deposits coming from your exhaust. Since these sensors are exposed to hot exhaust and can be corroded with carbon deposits from fuel and exhaust vapor, they can become damaged rather easily. When the oxygen (O2) sensor fails, it can cause the engine to misfire, lead to poor acceleration, bad fuel economy, and more. An O2 sensor failure will typically cause a vehicle to fail an emissions test in most US states. Other signs of a failing O2 sensor include:

  • Poor gas mileage accompanied by rotten egg smell
  • Rough engine idle
  • Engine misfires

What’s the solution to this problem? Depending on which sensor is damaged, the repair typically replaced the sensor. It is considered best practice to replace all O2 sensors at the same time. When one goes out, the others are usually not far behind.

4. Catalytic Converter is Clogged

While the O2 sensors monitor exhaust gases, the catalytic converter is designed to filter exhaust before it heads out the tailpipe. It converts harmful carbon monoxide into carbon dioxide. Like any other filter, the catalytic converter can wear out or become clogged with too much debris to be effective. When this happens, it will create an error code and trigger the CEL. A clogged catalytic converter can cause poor fuel mileage, engine misfiring, and even lead to internal engine damage if not replaced. More signs of a failing catalytic converter are:

  • Poor fuel efficiency
  • Little acceleration when pressing on gas pedal
  • Vehicle refuses to start

How to fix this issue? Replace the catalytic converter.

5. Air Filter or Fuel Filter is Dirty

Routine service and maintenance are critical for your vehicle to operate correctly and efficiently. Besides having your engine oil and filter changed every 5,000 miles, replacing your air and fuel filters is nearly as important. These filters keep the fuel and air entering your engine clean, which allows your motor to burn gasoline properly. If the filters become dirty, your fuel system sensors will alert the ECU and illuminate the dreaded check engine light. Some signs that one of these filters is clogged are:

  • Hesitating, misfiring, or stalling engine
  • Hard to start or not-starting car
  • Performance problems at different speeds
  • Reduced fuel economy

How is this problem resolved? Simple – replace the air intake filter or fuel filter as recommended by your vehicle’s maintenance schedule.

6. Mass Air Flow Sensor is Damaged

You’re probably seeing a pattern here. In reality, most check engine lights are triggered due to fuel or emissions system issues. The mass air flow sensor or MAF is responsible for carefully mixing the right ratio of air to fuel, maintaining a clean and efficient burn inside the combustion chamber of your engine. If the MAF is damaged, the MAF sensor will send a signal to the ECU to let you know this needs to be replaced or cleaned. A faulty mass air flow sensor can prevent your engine from starting. If your car will crank over but not light-up, this is probably the source. On top of not starting, other symptoms of a failing air flow sensor include:

  • Engine stalls shortly after starting
  • Hesitating engine with a heavy load and/or during acceleration
  • Hiccuping engine

How to fix a mass air flow sensor issue? This is one of those problems that should be inspected by a professional mechanic first. They can determine whether the light is caused by a damaged MAF sensor or it simply needs to be cleaned.

7. Engine has Oil Lubrication Issues

While the top six reasons above can be easily resolved and diagnosed in most cases, this one is rather tricky. Your engine needs oil to keep all those moving parts cool as it revs up. Sometimes, dirty oil, engine sludge, or a damaged oil pump can cause oil pressure to reduce or increase higher than it should. When this happens, the oil pressure sensor will tell your ECU and illuminate the CEL. This is one of those situations that should be taken very seriously, as an engine not receiving the proper lubrication can overheat quickly and cause internal engine damage. It can also be possible that this sensor is damaged and sending false data.

Anytime the check engine light is on your dashboard, it’s something to be taken seriously. When this happens, don’t wait or assume it will just go away. This could turn a minor repair into a major mechanical expense. Contact a professional mechanic and have them complete a check engine light inspection, so they can determine what repairs are needed to give you peace of mind, and ensure your car continues to run strong.


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