* Power Down: The computer (PCM - Powertrain Control Module, and other modules) will lose power and shut down completely. This interrupts any ongoing processes.
* Loss of Radio Presets, Clock Settings, and Other User-Stored Data: These settings are stored in memory that's powered by the battery. They'll be lost.
* Possible Need for Relearning: The PCM stores various adaptive learning data (things like idle speed, throttle response). This data will be lost, and the vehicle may need a short driving period to relearn optimal settings. This usually involves a bit of rough running until relearned.
* Security Systems: Your car's anti-theft system might require reprogramming after reconnecting the battery (this varies by system; consult your owner's manual).
* No "Factory Reset": It will *not* erase fault codes (unless the codes are stored in volatile memory which is less common) or factory settings. Diagnostic trouble codes (DTCs) will likely remain in the PCM's memory.
In short, disconnecting the battery is a hard power-cycle; it interrupts the computer's operation, clears volatile memory, and may require some relearning on the vehicle's part. It's *not* a true reset to factory defaults. If you're looking for a more comprehensive reset, you'd need to use a scan tool to clear codes and potentially reset parameters.
