The check engine light reset procedure can go down two paths. You can use a code scanner to retrieve and clear the codes, which is the proper method.
Or you could disconnect the battery and drain the memory holding capacitor in the computer to clear it that way. I will explain both for you, but I only recommend using a scan tool.
I just added an auto repair video explanation at the bottom. If you disconnect the vehicle’s battery this alone may not clear the trouble codes.
Some manufacturers store voltage in a capacitor to hold the computer’s memory. After the battery is disconnected you can hold down on the horn button to drain the small amount of electrical energy.
This circuit is hot at all times regardless of key position. Doing this will definitely achieve the check engine light reset you were looking for. However, there might also be some annoying side effects to the procedure.
The vehicle’s main computer (PCM) stores the diagnostic trouble codes and also holds what is called the learned memory for a few other important items.
The main ones are transmission shift points and the IAC (Idle Air Control) idle step motor position memory. If you reset the check engine light by disconnecting the battery you clear out these important memory functions.
Nevertheless, this isn’t something that can’t be overcome with some patience. The main on-board computer will have to relearn these import settings.
What this means for you is when you restart the vehicle you may have erratic idle and poor performance for several driving cycles after the check engine light resetting procedure was performed.
How to Clear Check Engine Lights
When you disconnect an automobile battery you may also run into trouble with theft deterrent radios and even factory installed car alarm systems.
Let me supply additional reasons why it’s not worth disconnecting the battery. An important reason for following the proper check engine light reset procedure is, you really want to know what the stored code is.
First of all, if you cleared the code by disconnecting the battery and there is a hard failure in the engine control or emissions system then the light will just pop right back on. Why cause yourself the troubles for no reason?
The best way to go is by using a code scanner to retrieve the code and then clear it. I was recommending the Actron 9125 for the most basic tool, but Innova has recently lowered the price on the color screen 3030g scanner. This is a lot of scan tool for the money.
In fact, it’s one of the most popular and inexpensive automotive scan tools on the net right now. You can find cheaper ones, but this thing has some useful features that others don’t include.
Sidebar: The Autel above is twice as much, but scans for air bag and ABS codes. Now that you know what code was set in memory the next step is to clear it and then drive the vehicle and see if the light returns.
If the check engine lamp comes back on and you retrieve the same code, then you know it’s something that needs to be repaired. You will also know in what system the failure is hiding.
If you have a good car service manual you can follow a diagnostic chart for the set code number. A service manual and a scanner used together is an efficient way of solving service engine soon light problems.
Auto scan tools have really come down in price since OBD II came out in 1996. Standardizing the engine diagnostics systems also simplified the operation of the tools used to diagnose malfunctions and the tree charts used to troubleshoot and repair them.
Benefits of Resetting the Light
The OBD II system is on the sensitive side and may set a diagnostic trouble code that doesn’t actually need to be addressed.
If you have a scan tool and you pull a code for a small leak in the evaprative emissions system, there is a chance that if you reset the check engine light this code will not return.
In this instance you have just saved yourself a trip to the dealership and maybe hundreds of dollars. On the flip side of this equation maybe you have set a code for an engine misfire.
Important Check Engine Trouble Codes
Some codes like the gas cap codes are considered nuisance issues, because they don’t stop you from getting to your destination or harm the car. However, the engine misfire is a different story.
If this problem isn’t addressed, not only will you have poor fuel economy, but you would fail an emission or smog test.
Unfortunately, that’s the good news. This problem could also eventually do damage and cause safety concerns if left unresolved.
When a cylinder misfires and sets a code you can get raw fuel laying in the catalytic converter that could eventually destroy it or ignite. This is why you want to know what’s stored in the computer memory.
Using automotive diagnostic scanners is a fairly simple auto repair skill to master in my opinion. All of the major brand tools come with instructions that walk you through the code reset procedures.
It boils down to plugging in the tool to the OBD II diagnostic connector which is usually located near the steering column on the bottom side of the dash panel.
You turn on the key and scan by pushing a button that says read codes. After writing down the numbers of the trouble codes stored you can then hit the erase codes button.
Now you’re done and ready to test drive the car. I wanted to tell you about one of the scanners that arrived last year. It is the Actron 9550 pocket scan on the right.
It’s a few bucks more then the 3030 above. It has a few more features and is much smaller in size with a longer cable. It can easily fit in a glove box. It will reset the check engine light with a button push.
Two advantages of the scanner is the snapshot mode and Data stream access. This has proven to be a big help for do it yourself car mechanics. There is a full review of this code reader on the Amazon website.
Tell friends with car problems about this page.
For more articles on this subject visit my main page about check engine light diagnosis and repair. If you own a laptop you have the option of performing detailed diagnostics with a special interface device that displays real time data on a portable PC. Learn more about car code scanners for laptops.
Common problems on certain vehicles are identified and documented in factory technical service bulletins. I have posted several popular ones on my page labeled auto repair news.
Auto repair video explanation of how your shop diagnoses check engine lights. Note: They do not mention cost.
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