Audi, like other modern car manufacturers, uses a computerised engine control unit (ECU) to monitor vital engine functions and quickly identify malfunctions. As part of the diagnostic process or to delete error codes, the ECU may need to be reset. This task can be completed in a few minutes even by someone of novice skill.
Disconnect the black battery cable. Adjust the wrench to the size of the nut and loosen it only enough for the cable to come off, then set the cable aside. There is no need to remove the positive cable.
Switch on the interior light for a few seconds to remove any remaining power from the Audi's electrical system. Wait 15 minutes before continuing to ensure the reset process has time to complete.
Slip the negative cable back on the battery and turn the nut with the wrench until it is tight enough to hold the cable in place. Do not over-tighten.
Start the ignition. The motor may run rough for the first few minutes as the ECU relearns the engine management settings.
Do not allow the negative and positive cables to touch, or for any tools to bridge the gap between them.