Editor's note: This post has been updated in March 2020 for accuracy and the latest information.
People usually get interested in the On-Board Diagnostics when the Malfunction Indicator Light illuminates on the dashboard of their cars. The Malfunction Indicator Light (MIL) is also known as the Check Engine Light. The purpose of this warning light is to indicate a detected problem with the car and alert the driver about the issue.
The OBD2 system illuminates the light when there is a problem with the car’s engine, transmission or emission control system. The light turns on only for a reason and you should not ignore it. You should always investigate the cause.
What does the Malfunction Indicator Light mean?
The Malfunction Indicator Light can signal three different types of problems.
Occasional flashes indicate temporary engine malfunctions. In this case, it is good to be aware of the possible forthcoming issues which can later on turn to more serious ones.
The most common case is when the indicator light stays on constantly. It indicates more serious problem that requires action to be taken; the sooner the better. Yet, sometimes the issues aren’t that serious but can affect the emissions of the vehicle, for example.
The most serious type of signaling is when the MIL flashes all the time. It is a sign that your vehicle’s engine is misfiring. The issue is a major one and you should stop the engine immediately to prevent serious damage. For instance, it might cause the catalytic converter to overheat and even cause a fire.
N.B. It is totally normal for the light to illuminate for a few seconds when you start the engine. But it should go out when the engine is running.
How to get more information about the issue?
Some of the OBD-II issues are relatively small. Those don’t have much impact on engine operation. On the other hand, some of the issues are major and need appropriate measures to be taken. Unfortunately, there is no way to distinguish between them by just looking at the MIL.
The only way to find out what’s wrong with the car is to plug in a OBD2 scantool. The tool and accompanied software can read the Diagnostic Trouble Code(s) from the system. Every time the OBD system turns on the MIL light, it will also store a Diagnostic Trouble Code (DTC) in the electronic control unit. The DTC will give information about the detected issue.
The OBD2 system can turn the MIL off automatically if the conditions for the problem stop to exist. After evaluating a system or component for three consecutive times without detecting the problem, the light can be turned off. But, usually the light remains on.
What are the next steps?
If you have a OBD-II scantool (for example one with ELM327 chip) and OBD diagnostic software, you can check the reason for the problem yourself. For example, our car diagnostic software can read the DTC code from the car and give you description of the code. Those will guide you towards the main cause of the problem.
With diagnostic software, you can even reset the MIL yourself. It is important to clear the Malfunction Indicator Light after fixing the problem. This is because, for example, the car will fail emissions testing if the MIL light is ON when tested.