When it comes to the reset check engine light process there are not as many options today as there where for cars built prior to 1996. Before the invention of OBD 2 resetting a 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 an automobile is newer or post mid 90's I Do Not Recommend that you disconnect the battery for clearing the service engine soon light. However if you 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 the Mitchell repair manual diagnostic ladder diagrams for the specific code set on that model. It's not expensive ( about 17 bucks ) and 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).
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 light the 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 any good if you bought it off a used car lot. Also note if your vehicle is equipped with a factory installed car alarm or active theft system disconnecting the battery may cause problems with this system also.
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 by Innova. Take a look at the customer reviews on this item. The equus 3100 can read the data stream and may be overkill if you just want to reset check engine lights. For people that want to keep it simple see the cheaper Equus 3030.
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 keep alive memory built-in to the computer system.
When you disconnect the battery from the vehicle to reset 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 electrical current that is stored in the control modules capacitor. The horn circuit is a hot at all times circuit. It's alive regardless of where the ignition key is! This will reset check engine light on most models.
When you do this you'll be erasing all memory learned by the computer. When you reconnect the battery on the vehicle and confirm the reset check engine light procedure has been successful you may find the engine idle erratic for several drive cycles. This is because idle learn data has been erased along with the 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. To the right is one of the cheaper scanners on the market. The equus 3030 works great but It reads and erases codes only. The tool communicates with all OBD II protocols from 1996-2011.
This is much better then disconnecting the car battery. Using the automotive scanner tool will avoid any and all of the above-mentioned problems. If you are interested in a scan tool that can do more you can visit the automotive scan tool reviews page.
In the auto scan tool section you'll find more articles about the diagnosis of check engine lights. Plus more on the reset check engine light procedure using automotive scanners.
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