Does Disconnecting a Car Battery Clear the Computer?by Laurie Brenner
As of 1996, all cars sold in the United States have come equipped with an on-board diagnostics computer system that retains error codes and engine settings in its memory. Some have the ability to retain the code up to several hours after disconnection from the battery, depending upon the vehicle model and computer capabilities. But in most vehicles, once the battery is disconnected, the car's computer memory clears. While this is an easy fix to get rid of codes, it’s not advisable unless you have corrected the problem with the vehicle first.
Use a settings keeper that will save computer codes, radio settings and other settings normally lost when disconnecting the vehicle’s battery. Most of these small units plug in to the cigarette lighter or the optional 12-volt plug-in port on many vehicles. The unit will maintain the on-board diagnostic computer settings, radio settings, alarm theft deterrent settings and any other personal settings when you have to replace or complete maintenance on your battery and disconnect it from the vehicle.
Check Engine Light
When you first start your vehicle, the check engine light flashes and then turns off after the engine engages. This is an indication the on-board diagnostics computer is functional and working. If the light comes on and doesn’t go off, something in the system is indicating a problem. Sometimes this happens after disconnecting the battery; the computer needs to complete system checks and “relearn” the vehicle's operations. This may take several miles and a few trips to complete.
OBD Readiness Status Indicators
All on-board diagnostic computers have an OBD readiness status indicator, which means the computer monitors sensors and conditions in the vehicle and delivers a "ready" that indicates all is well in certain systems. Depending on the model and vehicle manufacture date, the computer completes up to 12 checks. When testing emission, for example, the computer has a “ready” or “not ready” status. When the computer’s memory clears, it will take up to 50 miles and a few trips for the computer to complete all the checks and provide a “ready” status or throw an error code. Relearning the fuel values may take up to 500 miles, depending upon the vehicle. When the system is in "not ready" status, it will not pass the OBD-II part of the smog emissions tests.
If you want to find out what error codes are coming up with your computer, or need to verify why the check engine light comes on, rent or purchase an OBD-II scanner to check the codes and determine the problem. Some auto parts stores will also complete the check free. It’s important to keep the settings in the computer so you can determine any vehicle problems. Disconnecting the battery to remove the settings isn't advised; it doesn’t correct the problem and the codes will reset once the vehicle completes the necessary trips and miles for the on-board computer to deliver its diagnostics.