I HAVE GOT CODE D3

Audi and other suppliers only sell complete lights.
Try you luck at a scarpyard or ebay.
These lights are expensive.

If you have facelift version is a sealed unit.
These is even more expencive.

Pre facelift is not sealed, easy to fix.
Often the LED`s burn out. These can be replaced if you feel safe with a soldering iron.

Check this out if you have pre facelift:
https://www.audiworld.com/forums/a8-s8-d3-platform-discussion-60/d3-a8-led-tail-light-diode-repair-2901254/

press the helpful button
http://repairpal.com/what-does-service-code-d3-mean-219

if the fuel filter is blocked that puts alot of strain on a fuel pump or check that you have a good earth connection

I would start with a tune-up: new plugs and wires, new distributor cap/rotor button, new air filter, maybe a new fuel filter. If you get gas at the same location all the time, try a different one for a while too.
These would be the easiest to do first to eliminate some common issues.

define alot, that word means nothing
post range per gallon, MPG.
or post Liters per 100km
or >?
only data makes senses.
by pick the most expensive part on car to change out. on a guess.
post your MPG.

here are all thing for low mpg.

start at measurement errors. or stolen fuel.

http://www.fixkick.com/MPG.html

P1406 EGR Valve Pintle Position Circuit Fault
This code is usually caused by carbon buildup in the EGR valve. The carbon deposits prevent the valve from closing completely.

After removing the EGR valve, clean the pintle seat area of the valve and the corresponding passageway in the engine.

or simply replace the EGR valve.

The purpose of you EGR Valve is to recycle un-burned fuel back into the engine, not only improving fuel economy by reducing the engines pumping losses, the EGR increases the mass in the intake; more mass means higher pressure, or less vacuum. Now the piston has less resistance during each intake stroke, which results in better gas mileage.

So if your EGR Valve is clogged, damaged or simply just not working - the those rich un-recycled fuels will make your car burn richer. (Which will make you burn more fuel.)

Hope this helps!

use d4. d4 is for normal driving. d3 is to down shift the transmission in case you wanna pass. use d3 only if you want to down shift because if you leave your car in d3 on the highway, it doesnt allow the 4th gear to engage, causing your engine to run at higher revs, not only wasting more gas but also making your engines life last less. hope this helps. if you need more help, let me know. goodluck

some people will tell you that d3 is for town and d4 for the open road,that is totally wrong.D3 is for engine braking when going down a mountain(saves the engine/gearbox doing too many changes)so leave it in D4 for everyday use.....also saves you fuel..

Well every gear has its limitations. I am not sure what the mph ranges for each gear are, but i've gotten to 60+ mph in 3rd gear. The only thing about driving on the freeway in 3rd gear is that it is alot less efficient mileage wise. Your turning more RPMS while on the freeway in 3rd gear so you burn up alot more gas.
Driving in D3 is driving in 3rd gear. It could burn up the tranny but it depends on the vehicle itself. It is not really made to do highways speeds in 3rd gear. It should be in 4th or 5th (OD)

d will do just fine but most say city driving use d3 but at higher speeds you will burn more gas because the car will rev higher