Check Engine Light

            The Check Engine Light or Service Engine Soon Light was originally to inform you if there was a problem with the emissions system of your vehicle. In today’s vehicles the Check Engine Light may illuminate if there is a problem in your emissions system, fuel system, ignition system, charging system, and in some cases your automatic transmission. Can your vehicle be driven if Check Engine light is on? Yes, as long as it is not flashing.

            When your Check Engine Light illuminates we can install a scan tool or laptop computer and retrieve diagnostic codes. Now diagnostic codes are just that, Diagnostic Codes! They do not tell you exactly what is wrong with your vehicle, they tell you what particular system is having a malfunction. Then a more thorough diagnostic routine will need to be performed in order to determine exactly which component has failed. We have seen many cases where a customer will have someone retrieve the diagnostic codes then replace several components spending several hundred dollars and never fix the concern. For example one of the first ones that comes to mind is the code retrieved for EGR (exhaust gas recirculation valve). Many customers will replace the EGR valve, pressure feedback sensors, etc., only to find out that the problem was that carbon deposits had stopped up the EGR ports in the intake manifold. Or they may get a oxygen sensor lean code and replace the oxygen sensors only to later find out that problem was caused by a vacuum leak or maybe even a restricted fuel filter.

             Now how does the on board computer know when to illuminate the Check Engine Light? Well, contrary to popular belief, computers are not very smart. They can only do what a programmer has told them to do and they can only do one thing at a time. However they can do this very efficiently and very, very fast. Now a program within the computer will monitor a signal from a sensor or sensors and will activate a relay or solenoid based on this signal to control a function. It will then monitor a reference voltage from this component to make sure that that voltage stays within a certain range programmed into its memory.  It will then continually monitor that voltage from each component that it is controlling, one at a time, over and over. If it sees a voltage go out of range two consecutive times it will illuminate the Check Engine Light and store a diagnostic code in memory. The light will remain on as long as the problem is still present but will turn off if the problem is not present; however, the diagnostic code will remain in the computer’s memory for several drive cycles-- usually a few weeks or possibly a few months

.            In today’s vehicles when a diagnostic code is stored the computer will make a snap shot of the conditions of the vehicle at the time the fault occurred-- like engine temperature, vehicle speed, throttle position, etc.; this is called Freeze Frame Data. This information enables us to recreate the conditions when the fault occurred-- a very handy diagnostic aid. However this information can not be accessed with a standard code reader and is erased when the trouble codes are retrieved and erased. We have seen many customers take their vehicles to a local parts house and have them retrieve the codes then bring the vehicle to us with a print out of the code thinking this will help speed up the diagnostic time and save them some money. On the contrary it actually doubles the diagnostic time because we did not have this information. 

             Also in today’s computers there are several monitors that must be completed. Basically this is a program that runs a check on a system-- like your antivirus or security software on your home computer. These programs will test a particular system and if it sees a fault will also illuminate the Check Engine Light. These monitors are what must be completed and passed in order for your vehicle to pass the North Carolina State Inspection. These monitors are reset whenever a repair is done to correct a fault causing your Check Engine Light to come on. The process to complete these monitors requires that the vehicle be at a certain temperature-- the ambient air must be at or above 40 deg. F, fuel level must be between ¼ to ¾  of a tank, and you must drive at several different speeds for a certain amount of time(usually around 20 miles of driving). If the ignition key is cycled off before the monitors complete, then you must wait for a 6 hour cool down before they will try to complete again. These monitors should be completed in order for the technician to be totally sure that the repair done has corrected your problem. Many times we will get a vehicle in for a Check Engine Light repair and it will have almost no fuel in the tank, so it makes it very difficult to confirm the repairs are done.

                We hope that this will give you a basic understanding of the Check Engine Light.