Why Does a Check Engine Light Stay On

The check engine light can come on for many reasons ranging from a loose gas cap to bad spark plugs or a serious engine or transmission problem. 

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 the On-Board Diagnostic (OBD) system in your car detects an engine problem, emission leak, or a transmission problem.

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


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. 


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. diagnose check engine light
  2. Turn on ignition. Turn the unit's power on. Some run on the vehicle's power; some do not. diagnose check engine light
  3. Select Diagnose by pressing OK/Enter/Read Codes. Next, make sure to select Read Fault codes.diagnose read reset checke ngien light with an obd2 scanner
  4. Write down all the fault codes. Once you get the code, fix the problem, then clear the code. If you reset the codes without fixing the problem, the check engine light will come back on. 

Got the code; what's next? 

check engine light on, engine shaking

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.

You need to keep in mind that even though the fault code may indicate that the problem is with your oxygen sensor, replacing the sensor may not fix it. 

Check Engine Light Flashing

If the check engine light is blinking or flashing, you have an engine misfire. A misfire is caused when the fuel is not getting burned in one of the cylinders.

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. 

Continuing to drive when the check engine light is flashing can damage the engine and catalytic converter.

Understanding Fault Codes

how to scan obd ii codes using an obd ii code reader

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 includes the ABS, 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 specifically 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 most helpful to refer to a database of fault codes to discover the specific problem or use an online code interpreter.

Frequently Asked Questions

Can I diagnose the check engine light myself? 

Reading the codes related to the check engine light is easy. An OBD-II scanner will allow you to read those codes quickly. These scanners work on all vehicles manufactured from 2000 to the present.

How is an OBD2 scanner used? 

The scanner is plugged into the OBD port of your vehicle, and it subsequently will display what the trouble code is. From there, you can research the code and discover the part of the car where the problem is occurring, then take the necessary steps to repair it.

Can I read check engine light codes for free?

If purchasing a scanner is not an option, but you still would like to self-diagnose the problem, many automobile repair shops or retail stores (such as Auto Zone) have a code reader for you to use. They often will read the code stored in the vehicle's memory for free.

What are alternative names for check engine light? 

Depending on the manufacturer, the warning on the dashboard may display one of the following: 

  • Service Engine Soon
  • Check Engine
  • Check Engine Soon
What is an OBD-II Scan Tool?

An OBD-II scan tool, also sometimes called an OBD-II scanner, is a hand-held device that plugs into your vehicle's OBD-II port and connects to the vehicle's electronic control unit (ECU), which is essentially a computer.

All vehicles manufactured in 1996 or later have OBD-II ports. When the scan tool is connected, it will retrieve any error codes that have been stored in the ECU memory.

When do I Use the OBD-II Scan Tool?

When an error code (its formal name is Diagnostic Trouble Code or DTC) is stored in the memory of your vehicle's ECU, it will cause your check engine light to illuminate on your dashboard. Usually, it is an orange light in the shape of a car engine that might say Check Engine, Check, MIL, CEL, or Service Engine Soon. 

It is also known as the Malfunction Indicator Light (MIL). It means that something is wrong with your vehicle's operation. It could be a simple fix or a more complicated problem. The answer to finding out exactly the problem lies in retrieving and then interpreting the error code stored in the ECU.

By  YOUCANIC Automotive Experts

Published on: Tuesday, December 10, 2019.
Updated on: Wednesday, December 23, 2020