The Check Engine Light explained

Starting in 1996 all passenger vehicles and light duty trucks produced or imported for sale in the U.S. had to be OBD II compliant (On Board Diagnostics two). As well as meet newer vehicle emissions regulations. Part of this technology involves the check engine light or MIL. (Malfunction Indicator Lamp). While it’s true that prior to 1996 the check engine light did exist on vehicles, it took on an entirely new meaning and purpose in 1996.And this is where the confusion starts.

The check engine light comes on when the ECM (Engine Control Module) recognizes a malfunction with an emissions control device or an emissions control device monitoring system or if a specific condition is not being met. In many cases the vehicles overall performance won’t change much if at all when the light comes on, and is thus not taken seriously by the majority of car owners. But what the light represents is a potential failure in one or more of the vehicles systems that are meant to control the level of tail pipe and evaporative emissions produced to acceptable levels. In some cases the light can come on when the gas cap is loose. But the light can also come on when the catalyst system has failed.

Normally neither one will adversely affect the vehicles performance but both will allow the vehicle to excessively pollute our environment. The gas cap is meant to keep the fuel vapors sealed in the tank and evaporative emissions system rather then allowing the fuel vapors to enter the atmosphere as hydrocarbon emissions. The catalyst system is designed to lower tail pipe emissions to acceptable levels. These are just two examples of many possibilities.

There are essentially two types of OBD II Fault codes, conditional and component. A component code usually indicates a problem with a particular device, its circuitry or controls. A conditional code is set when a specific condition triggers the ECM to Command on the check engine light. Conditional codes are the hardest to accurately diagnose and repair and are typically the ones that do the environment the most harm. It is almost impossible to accurately diagnose and repair a conditional code with a code reader from a local parts house. It is simply not enough to have the code number but rather a thorough analysis must be performed by a qualified professional technician.

When a code is cleared, the emissions systems monitors are also turned to not ready status. It takes a specific drive cycle to return the monitoring systems back to ready status and the whole time the monitors are off they are not monitoring the emissions control devices. This is why sometimes a code can be cleared and not come back for weeks. Until the monitoring system is back to ready status the ECM doesn’t know about a device or a condition.

Specializing in Subaru repair gives us an advantage over other general repair shops when it comes to the diagnoses and repair of the check engine light and vehicle emissions systems on your Subaru. We will accurately diagnose the failure, any potential causes of the failure and keep you informed the whole way through. More importantly we can provide you with tips to help reduce the amount of emissions your vehicle emits and keep you an informed Subaru owner.

The above was prepared and written by.

Justin Stobb

Owner All Wheel Drive Auto

ASE Master Certified Technician, L1 Advanced Engine Performance Certified

WA. State Dept. Of Ecology Authorized Emissions Specialist