What Does It Mean When Check Engine Light Turns On

The Check Engine Light (CEL) or Service Engine Soon (SES) can come on for many reasons, including a loose gas cap, bad spark plugs, dirty mass airflow sensor, faulty oxygen sensor, serious engine issue, and failure of transmission.

In this guide, you will learn what the check engine light means, common problems, and how to diagnose the check engine light yourself.

What does the check engine light mean?

Check Engine Light comes on when your car’s On-Board Diagnostic (OBD) system has detected an engine problem, emission leak, or transmission problem.

A fault code will be present in the Engine Control Unit (ECU) memory and easily retrieved with an OBD2 scanner.

Symptoms

When the check engine light comes on, you will notice at least one of the following symptoms:

• Increased fuel consumption
• Poor acceleration
• Engine shakes
• Rough idle
• Engine shakes, misfire present.
• Transmission stuck in limp mode.

Common Causes

This is a list of common problems that trigger the check engine light.

• Loose gas cap
• Spark plugs
• Ignition coils
• MAF sensor
• Vacuum leak
• EVAP leak
• Oxygen sensor
• Catalytic converter
• Transmission problem

To find why your check engine light is on, read the fault codes with an OBD-II scanner.

Should I drive with the check engine light on?

If the check engine light or service engine soon stays on constantly, you can drive your vehicle home or to a shop in most cases. Before you continue your journey, ensure the vehicle is not overheating, and the oil light does not come on. Note that the transmission may get stuck in 2nd or 3rd gear, and speed may be limited to 40 mph maximum.

Why is my check engine light flashing?

If the check engine light or service engine soon starts to flash, a misfire has been detected and one or more cylinders are not working properly. This problem is often caused by a bad spark plug, mass airflow sensor, ignition coil, or clogged catalytic converter.

The engine will also shake a lot. Do not drive the vehicle if the check engine light is flashing or causes more damage to the engine. Call your mechanic or dealer and ask for advice.

A misfire can be caused by:

• Bad spark plugs
• Faulty ignition coil
• Clogged catalytic converter
• Blown head gasket
• MAF sensor
• Vacuum leak
• Fuel issue
• ECU problem

A misfire is a critical problem that needs to be addressed right away. If this happens, find a safe place to pull over. If the engine is overheating, you should call a tow truck instead of driving.

When the check engine light is flashing, continuing to drive can damage the engine and catalytic converter.

How to diagnose the check engine light

Follow these instructions to read the fault codes.

For this procedure, you will need an OBD-II scanner. Some scanners cost as little as $20. You can buy a scanner online or check with your local auto parts store to read the codes for free.

1. Plug your diagnostic scanner into the port under the dashboard located under the driver’s side. All cars 1996 and newer have this port. You have to slide under the dash to see it. 
2. Turn on the ignition. Turn the unit’s power on. Some run on the vehicle’s power; some do not.
3. Select Diagnose by pressing OK/Enter/Read Codes. Next, make sure to select Read Fault codes.
4. Write down all the fault codes. Once you get the code, fix the problem, then clear the code. The check engine light will return if you reset the codes without fixing the problem.
   

Got the code; what’s next?

Sometimes the fault code may point you to the exact problems; other times, the code will only be the starting point.

Always perform further research and troubleshooting. Search the code online, plus your vehicle make and model. This will allow you to learn about common causes and solutions for your particular car.

Consider all possible causes.

Remember that even though the fault code may indicate that the problem is with your oxygen sensor, replacing the sensor may not fix it. For example, the issue could be a bad catalytic converter or a damaged wiring harness. You still want to first start with the least expensive and most probable repair. That is what a mechanic or dealer would also change first.

Understanding Fault Codes

The OBD-II code consists of five characters.

It begins with a letter and is followed by four numbers.

• P stands for powertrain (engine & transmission), B stands for the vehicle’s body (which includes A/C and airbags), C stands for chassis and the ABS, and U stands for the user network, including the wiring bus.
• The first number will be a 0 or 1. 0 means the problem is generic, while 1 indicates that the problem is manufacturer-specific.
• The second number will be between 0-9 (and may also have an A, B, or C if you have a hybrid vehicle). Here is a further breakdown of this placeholder digit:
• 1 and 2 refer to the fuel & air metering system, with 2 referring to the injector circuit.
• 3 refers to the ignition system or misfire.
• 4 means the problem is with the auxiliary emissions controls.
• 5 refers to the vehicle speed control and vehicle idle control system.
• 6 refers to the computer output circuit.
• A 7, 8, or 9 means the problem is with the transmission.
• The last two numbers describe the specific fault in the location specified in the previous number. It is helpful to refer to a database of fault codes to discover the specific problem or use an online code interpreter.

Frequently Asked Questions