Deceptive Lights: Check Engine Light

Everyone out there has probably had a car that had a check engine light come on at some point. Some of you might have even had one blink at you furiously. Sometimes, the illumination corresponds perfectly with the engine running rough, fuel economy falling into the basement, or the transmission shifting funny. Sometimes nothing seems different in the way the vehicle runs at all. Either way, anytime the light illuminates it means something is broken. However, when the check engine light illuminates, it probably doesn’t have anything to do with the engine itself. This can make the check engine light deceptive.

This little light of frustration and angst will illuminate anytime the engine control computer sees a failure in the powertrain controls that will cause an increase in vehicle emissions greater than 150% of the federal test standard. This means if something breaks and it causes your car to potentially pollute more than normal, the computer will turn the light on. Most of the time this failure is electronic in nature, and not mechanical.


The check engine light varies somewhat from one vehicle to another. Federal law dictates what it can be, but there are several options. The light must be amber (yellow-ish) in color. The light may say “Check Engine” or it might say “Service Engine Soon.” It may appear as a silhouette of an engine alone, or it make appear as a silhouette of an engine with the word “check” or a strange lightning bolt arrow in the middle.
Many old cars from the early days of computer controls may have used something completely different for the check engine light. On these old cars every manufacturer could do whatever they wanted when it came to this malfunction indicator lamp. Honestly, most cars back then were not smart enough to illuminate whatever they used for a check engine light when the vehicle experienced most failures, so the check engine light was not a very useful thing.

Federal laws regulating exactly what the check engine light should look like and exactly when and how it should illuminate didn’t become standardized until the 1996 model year. These new standards which are still in use today are known as OBDII. These standards came about to make things such as check engine lights simpler for vehicle drivers to understand, and to make the problems for which they illuminate easier for technicians to diagnose.

The most common failures that cause the check engine light to illuminate relate to the engine not running right at its most efficient. This inefficiency can cause rough running, stumbling, hesitation, lack of power, poor fuel economy, and of course increased emissions. Sometimes the engine will not seem to run any differently than normal, but this is not always perceptible to the driver, so if the light comes on it must be diagnosed.
Many times you will take your car into the shop to have the check engine light diagnosed and the technician may not be able to figure anything out right at that moment. Rather than educate you on the situation and the nuance of intermittent problem diagnosis, they may leave you with the impression that nothing is really wrong and sometimes lights like this just come and go. This is never true. If the light is really coming on for no reason it's because something is broken. If the light is coming on for a reason then something is broken. No matter how you slice it something is wrong. 

Diagnosing an intermittent failure that causes the check engine light to illuminate can be difficult and can take time. Be patient with your mechanic and understand that the time they spend working on this issue should not some free. Their time is their livelihood and you must pay for it even if it seems as if progress comes slowly. You would be patient with a doctor trying to figure out a physical ailment so you ought to show the same patience with your mechanic, after all, he can't ask your car where it hurts.

The check engine light must also not be confused with service monitors. Many vehicles have systems that can monitor the time and mileage intervals that elapse between vehicle services such as oil changes and tire rotations. Some of these monitors are very sophisticated, and some of them simply count the miles for you. A service indicator might say something such as “Service Vehicle Soon” or you might see a small illuminated wrench. To make things more confusing the vehicle that uses the service vehicle soon indicator might also have a check engine light that says service engine soon. These are totally different but can be really confusing if your vehicle has both of them.

Consult the owner’s manual for a proper explanation of the maintenance monitors and the associated indicators. The manual will tell you how they work, and it will even tell you how to reset them most of the time. The check engine light on the other hand has no simple reset procedure. If it comes on, something must be repaired.

The one thing that is usually certain is that the illumination of the check engine light has nothing to do with the engine itself. Some people worry about the pistons and valves and other mechanical components of the engine when this light comes on, but most of the time these things are fine…most of the time. 

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