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Does a Car Have to Relearn Idle When a Battery Is Disconnected?

by Lexa W. Lee

Many newer cars with powertrain control modules (PCMs) have to relearn engine idle after a battery is disconnected. The removal of electrical power from the engine erases idle memory from the PCM, the computer where it is stored. The relearning process depends on the PCM.

Idle Speed

Engine idle refers to the lowest rpm or speed at which the engine turns over when the driver is not depressing the gas pedal, according to At idle speed, the engine generates its lowest emissions, thus conserving fuel. Idle speed that is too low can stall an engine. If it's too high, fuel is wasted and more emissions are produced.


During the relearning procedure, the PCM may change the idle speed until the correct speed is achieved. If the engine threatens to stall or idles too high, stepping on the gas slightly or taking the car for a short drive will help set the PCM correctly. But with some PCMs, the relearning procedure is not noticeable.

Natural Memory

Engine idle is only part of the natural memory, or accumulated data, lost when the battery is replaced. Adjustments to the fuel mixture, transmission shifting and various habits of the driver may all have to be relearned. It may be necessary to use a scan tool to reset PCMs on newer cars and to reset the anti-theft system, according to

About the Author

Lexa W. Lee is a New Orleans-based writer with more than 20 years of experience. She has contributed to "Central Nervous System News" and the "Journal of Naturopathic Medicine," as well as several online publications. Lee holds a Bachelor of Science in biology from Reed College, a naturopathic medical degree from the National College of Naturopathic Medicine and served as a postdoctoral researcher in immunology.

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