How to reset check engine light

When it comes to the reset check engine lightprocess there are not as many options today as there where for cars built prior to 1996. Before the invention of OBD 2 resetting your check engine light was a simple matter of disconnecting the battery for more then 10 seconds.

OBD II was deployed in 1996 and changed the way codes are set and stored in the on-board computer. If your vehicle is newer or post mid 90's I do not recommend that you disconnect the battery for resetting the service engine soon light. Stick with me I will explain how to reset check engine lights by disconnecting the battery.

Yes, there is a trick for draining the capacitor that holds power for the electronic control module stay alive memory when power is disconnected. As a Certified professional auto mechanic it's my duty to let you know what kind of problems are common when you perform this operation! 



Remember if you have a hard failure the light will come right back on. Consider pulling the codes and following available online factory that provide diagnostic charts for the specific code set. This is how professional mechanics solve car problems.

Another reason for not disconnecting the battery is that newer vehicles have a few installed options on-board that require constant battery voltage. I know what your thinking, what if you have to replace the battery or disconnect it for cleaning.

I personally use a device called a battery tender. This is plugged into the cigar lighter socket before you disconnect the power and ground cables to not only hold the radio station presets, clock settings but also keep the on-board memory alive for the power-train control module (PCM).


Reset check engine light complications


For example if you have a theft deterrent radio and you disconnect the battery your radio may stop working when reconnected. When the power is disconnected to reset the check engine lightthe cars radio might think that it is being stolen.



The radio can go into a self defense mode and not operate until a special code has been inputted. In most cases this special code is included in your owner's manual. Or was supplied by the dealer at the time of delivery.


This may not do you any good if you have purchased the automobile in question off a used car lot. Also note that if your vehicle is equipped with a factory installed car alarm or theft deterrent system disconnecting the battery may cause a few problems with this system as well. 



The owner’s or service manual may provide a special procedure for replacing the battery and disabling the alarm system before the battery is disconnected. Some car makers provide a built in sleep mode for just this operation.

Last but not least stored in the computer along with the code is the automobiles learned memory. This stores driving habits like shift points and IAC steps for idle control. The vehicle may have stalling and hard shifting problems for a while after the battery is reconnected. It will usually recover but it may take several drive cycles for this to happen.



An easier way to clear the check engine light codes is to use an auto scan tool. The automotive scanner comes with good documentation that will walk you through how to retrieve the codes and reset the check engine light.


They can do this without disconnecting the battery power or causing any of the problems mentioned above. One of the best selling automotive scanners right now at the time of this writing is the Actron. For people that want to keep it simple see the cheaper equus 3030.


Reset check engine light the hard way

disconnecting battery image
If you decide that you have what you need to disconnect the battery and would like to reset the check engine light by doing so there are a few more tips that I can provide. Most vehicles from 1996 and newer have a stay alive memory built-in to the computer system.


When you disconnect the battery from the vehicle toreset the check engine light. The computer will be able to hold the memory and the code for several minutes and in some cases several hours.

Battery voltage is stored in a capacitor and is supplied to the computer to keep the internal memory alive. The way around this is to disconnect the battery and then hold down on the horn button. The horn circuit is one of only a few that is hot at all times (regardless of key position). This is why you can blow the horn without the ignition turned on. 



Holding down on the horn button will drain the small electrical current that is stored in the control modules capacitor. This will reset check engine light on most models.


As a side note, when you do this you will be erasing all memory associated with the computer as well. When you reconnect the battery on the vehicle and confirm the reset check engine light procedure has been successful, you may find that the engine idle may be erratic for several drive cycles. This is because the memory idle learn has also been erased along with your check engine light code. 



This problem is usually short-term and will correct itself after driving the vehicle for a few miles at highway speeds. Also note if the check engine light comes back on after this reset check engine light procedure you have a hard failure code. This is when a failure is constant and will need diagnosis.

So to review you do have a few options for resetting your check engine light, but the easiest way is to use an auto scan tool to clear the code.

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