What to do When the Check Engine Light Stays on

by Jody L. Campbell

Diagnostic trouble codes (DTCs) are regulated by sensors and electronically monitored circuits and valves. There are literally hundreds of reasons why a check engine light may illuminate on the dashboard. Designed to alert the driver that a problem has occurred with a component or components in the vehicle, ignoring it or just resetting the light can lead to future trouble and more expensive repairs.

Before and After OBD II

When the check engine light comes on, something triggered it. Prior to 1995 and 1996 when On-Board Diagnostics II was implemented with the intent to standardize computer diagnostics, the procedure to determine what the DTC that occurred was a little more complicated. Diagnostic Link Connectors (DLCs) could be located in a variety of places and often times, different adapters to connect to the links were required, depending on the make and model of the vehicle. In some applications, determining the code required a specific procedure of turning the ignition key to the key on-engine off mode and counting the number of blinking lights. OBD II was integrated to simplify the process and better serve both the driver and the mechanical diagnosis of the problem. Although not entirely foolproof, OBD II diagnostics standardize the computer attached to the DLC in order to determine what the DTC problem that has occurred. From there, simple diagnostics can be determined or it may also require further investigation.

Read the Code Before Erasing It

When a DTC had triggered the check engine light on the vehicle, it is trying to communicate a problem in a sensor or electronic module of some sort of failure. No matter what year the vehicle is, if it is equipped with an electronic control module or powertrain control module, the sensor or component that has failed or is compromised has communicated its fault to the computer of the car. Older vehicles can reset the light by simply unplugging the negative batter terminal for a period of 10 to 15 minutes. Newer vehicles with theft alarm deterrents can complicate the procedure. By disconnecting the battery, you can erase the theft deterrent codes and make them ineffectual. By erasing the code(s) without determining what caused them in the first place, you may be gambling with more expensive repairs in the future of your vehicle. DTCs happen for a reason. There are two types of DTCs coined in the automotive repair industry. One is a hard code and the other is a soft code. Hard codes will either erase for a short period of time until the computer has relearned itself and it will re-trigger the check engine light as soon as the IM monitors have relearned the problem. Other hard codes may prohibit the light to go out at all. These types of codes require diagnosis and repair to eventually correct the problem and keep the check engine light from re-illuminating. Soft codes can occur from something as simple as a loose gas cap or filling your gas tank with the engine running. A small vacuum leak is detected in the evaportive emission control system and fixing or not committing the same offense may keep the light from re-illuminating after reset. However, some soft codes may be trying to communicate that there are indeed parts that are compromised. Resetting the code(s) without determining what they are can be both detrimental to the vehicle and the owner.

Resetting the Check Engine Light

Prior to OBD II standardization, different adapters and DLC locations may vary widely. Often times, this will incur charges because the diagnosing facility has to obtain expensive equipment in order to diagnose correctly. OBD II systems are a little easier to reset. They require a simple hand-held code reader/resetter device that plugs into the DLC employing the same plug connector no matter what make or model vehicle you own. The code can be determined in addition to whether or not it is a hard code or soft code. Soft codes can be reset, but should be documented in the event they return. Hard codes should always be determined and may very well re-trip the check engine light almost immediately.

Driving With a Check Engine Light On

Because of the hundreds of different reasons that can trigger a check engine light, it's hard to determine how well the vehicle will perform when it comes on. This depends on what type of code or codes that occurred to offend the computerized system. If the light comes on, but the vehicle seems to be running okay, you can most likely continue to drive it. However, ignoring the light may lead to more expensive repairs to other components that would not have had to be replaced if diagnosed and treated early in the warning system. Hard codes can compromise the performance of the engine and the vehicle and may not allow the vehicle to run at all. By reading the codes and diagnosing the problem and then repairing the problem, you can save yourself some time and money before the problem compounds into an even larger one.

About the Author

Jody L. Campbell spent over 15 years as both a manager and an under-car specialist in the automotive repair industry. Prior to that, he managed two different restaurants for over 15 years. Campbell began his professional writing career in 2004 with the publication of his first book.

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