Popping sounds from the rear of the vehicle may be heard moments after starting the engine, or when slowing to a stop. This noise is from the leak detection pump filling the fuel tank with air, and releasing to retest the system when it finds a fault. The sequence of faults identifying this issue are as follows:

Succession of loud pops from rear of vehicle

Pops go away, and fuel economy decreases

Check engine light illuminates

OBD trouble code P2404 and/or P0441is stored

Engine will not start after refueling (must crank for over 30 seconds)

Vibration when slowing to a stop

The cause is a valve which is mounted on the front of the intake manifold, commonly called the N80 valve, or EVAP system purge valve. The valve is designed to open under specific circumstances, but a worn valve will remain open at all times, preventing the pressurization of the fuel tank. Correction of the issue is to replace the N80 valve with the latest revision.

Some Audi A6 Quattro owners report experiencing intermittent starting issues despite having no signs of battery or alternator failure. These problems may stem from a faulty starter motor, which can produce clicking sounds or fail to engage properly when starting the vehicle. Additionally, some owners find that a weak key fob can lead to inconsistent starting, as the signal may not be properly received by the car. Engine-related issues, including fuel system or ignition system failures, are also cited as potential causes of the starting problem. Furthermore, pump failures, such as a malfunctioning fuel pump, can prevent the engine from receiving adequate fuel, resulting in unreliable starting. It is advisable for owners experiencing these issues to have their vehicles inspected by a qualified mechanic or dealership for accurate diagnosis and necessary repairs.