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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. 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

2 Responses to “Your Subaru Check Engine Light Explained”

1. Craig Phillips Says:
   June 25th, 2008 at 8:31 pm

   I live in Crestline California and our community has a large amount of Subaru Outbacks on the road. Everyone that I know that has a 2000 has their check engine light on. We have had the codes checked and random issues that are not resolved even with replacing parts. When 6-10 people experience there is a design flaw.

2. Justin Stobb Says:
   June 26th, 2008 at 12:34 am

   Craig,

   Please remember I am unbiased and only trying to factually educate you and other readers.

   A check engine light can come on because the driver puts gas in the car and did not tighten the gas cap up. How is this design flaw? It is mandated by law by the Department of ecology.

   The check engine light is geared towards alerting the driver when the computer sees a problem with the vehicles emissions system and there are Thousands of possible reasons for the light to come on.

   Just as there are a thousand reasons you might have a headache but bottom line the only thing you know is that you have a headache or a check engine light is on. If taking aspirin doesn’t fix your headache is your body a design flaw? Or was your headache not diagnosed properly or maybe there are a number of things contributing to your headache as your whole body is aging at the same rate of speed. Just like every part in your car is the same age and guess what, things are going to fail in unison.

   If a check engine light comes on and a code number P0325 Knock sensor circuit is set, and then diagnosed by a pro, not a code scanner at your local parts store, the P0325 should be satisfied once the properly diagnosed condition is repaired causing the code P0 325. If the check engine light comes back on a week later and now its P0440 Evaporative emissions system leak detected that is a new problem that must be diagnosed by a pro, it may be the gas cap is loose but it may also be that the purge canister has a leak also.

   Due to how the emissions system works not every system is monitored by the vehicles computer every second the engine is running but rather when a specific event is met. You can look online for an OBDII drive cycle and see what I mean. You can ATTEMPT to fix a problem but it may be weeks or months before the computer actually has the proper monitor turned back on to ready status. Until the monitor is ready the computer does not have the ability to command on a check engine light based on a failed component or condition triggering a code.

   Typical problems with the 2000 Subaru Outback that will trigger a check engine light are a failed knock sensor, failed Catalyst system, failed front air fuel sensor, failed rear 02 sensor, loose gas cap.

   Every one of these items fails just as often on any other Japanese import as they are almost all made by the same vendor first of all and subject to like environments in which they can become damaged.

   If a check engine light is diagnosed and comes back on for the same reason aka a reoccurring PO325 then the diagnoses was wrong or incomplete. If the light comes back on and it is a different code it’s a different problem.

   The real problem is that the parts stores try to devalue the real skill and knowledge it takes to truly diagnose a car, as does franchised auto repair shops such as the Tune and Tire centers.

   If you take your Subaru to a Independent Subaru expert and your neighbors and friends alike the experience will be different or hopefully after reading this you may better understand it’s a complicated process sometimes not a design flaw.

   Do also remember that the goal is to try and clean up the environment.

   If you are driving your Subaru around with a check engine light on you may not be doing your part to help

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