Malfunction Indicator Light

Posted by Auto Doctor on November 7, 2011

Check Engine Light

It is not unusual for people to get interested in On-Board Diagnostic when the Malfunction Indicator Light (MIL) lights up on the dashboard of their vehicles. The purpose of the Malfunction Indicator Light, which is also known as Check Engine Light, is to indicate a detected problem with the vehicle's engine, transmission or emission control system and alert the driver about the issue. The OBD system illuminates the light only for a reason and it should not be ignored; the cause should always be investigated.

The Check Engine Light indicator 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. Although, 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 constantly. It is a sign that your vehicle's engine is seriously misfiring. The issue is major and the engine should be stopped immediately to prevent serious damage. For instance, the condition 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 a vehicle is first started and extinguish when the engine is running.

Some OBD-II issues are relatively small and don't really have much impact on engine operation. On the other hand, some of the issues are major and require appropriate measures to be taken. Unfortunately, there is no way to distinguish between them by just looking the MIL. The only way to find out what's wrong with the vehicle is to plug in a OBD2 scan tool and read the Diagnostic Trouble Code(s) from the system. Every time the OBD system illuminates the MIL light, it will also store a Diagnostic Trouble Code (DTC) in the engine control unit of the vehicle to give information about the detected issue.

It is possible for the OBD system to turn the MIL off automatically if the conditions for the problem cease to exist. After evaluating a system or component under test for three consecutive times without detecting the problem, the light can be extinguished. However, usually the light remains ON. If you happen to own a OBD-II scantool (for example one with ELM327 chip) and a OBD diagnostic software, you can check the reason for the problem yourself immediately. For example, OBDAutoDoctor gives you the DTC code and description that guides you towards the main cause of the problem. With diagnostic software, you can even reset the MIL yourself after fixing the problem. It is important to clear the Check Engine Light because, for example, a vehicle will fail emission testing if the MIL light is ON when being tested.

Below is a screen shot of OBDAutoDoctor diagnostic tool for OBD systems. From the image, you can see how the DTC, system and textual description of a trouble code are illustrated. You'll also see the button for clearing the trouble codes and the MIL.