General notes

WARNING: This page is about a different car, the 2004 Audi A6, 2003 Audi A6, and 2002 Audi A6. However, it is still accessible from the selected car via links, so may be relevant.

Switch off the ignition before connecting or disconnecting the battery, otherwise the engine control module may be damaged.
A voltage of at least 12.7 V is necessary for satisfactory functioning of the electrical components.
During some of the tests, a malfunction may be recognized and stored by the control module. At the end of all tests and repairs, the DTC memory should be checked and, erased.
If the engine starts and then stops after malfunction search, repair or tests on components, because the immobilizer is blocking the engine control module. The DTC memory must then be checked and, if necessary, the control module adapted.

CAUTION:

On vehicles equipped with Audi Telematics by OnStar ®, switch-off the emergency (back -up) battery for the Telematic/Telephone Control Module prior to disconnecting the vehicle battery. Switch-on the emergency (back -up) battery after reconnecting the vehicle battery. See EMERGENCY BATTERY, SWITCHING OFF/ON .
Always switch ignition off before disconnecting or connecting battery. Failure to do so may damage a control module.