I have just had, and resolved myslef, the same problem. Airspring leak. Few tips:
easy to diagnose, get it to highest setting, cover both airsprings in thick soapy water and look for leaks. Mine actually leaked constantly on lowest setting 1 and slowly (overnight) on level 2 not at all on level 3 or 4 so check for bubbles at all levels. Mine leaked where the airspring folds on itself (about half way down) and was really obvious. I tried to deny noticing the problem for a few months as it seemed intermittant but gradually worsened until I could actually hear air leaking all the time at level 1. Dont put it off too long as the extra wear on the compressor can burn it out and that is even mroe expensive.
Would not imagine its the level sensor if the car corrects its level once ignition is switched on, if it was the sensor it would ride wrongly all the time.
Advice from my Audi independent was to replace airspring(s) with Arnott rather than Audi, they come with lifetime guarantee. They are made in America, seem cheap from manufacturer but when postage & VAT is added they are no cheaper than buying via UK company called P38spares.com. I acutally got mine on e-bay from Germany for £250 plus £15 postage (arrived in 2 days).
If you feel like fitting yourself they are very much manageable as a DIYer, I found it rather easy & its very rewarding to have an allroad that no longer sinks however long I leave it that I have fixed myself. Arnott supply good picture instructions (you can download them from their own webpage) also good instructions at
Replace Front Air Spring although they are fitting Audi replacment not Arnott so the seals are slightly different. The only tricky bit is releasing the upper links from the pinch bolt!!! Mine were easy as I had replaced by CV boots only 2 months ago but releasing them then was a bu**er. The only other annoying step is the old spring needs unscrewing from the upper mount, in itself fine but the 6 bolts are 12 point torx rather than the normal 6 point. Make sure you have the right tools before you find your suspension over the garage floor and need your car to pop out to buy a set - I speak from experience there.
Any garage would advise replacing both fronts together as they tend to fail in pairs, but I have only replaced my worn out one for now, the other can't be long away. They are designed to ride exactly the same as the original so there is no problem in doing them 1 at a time.
if you do your own PUT THE CAR IN JACK MODE FIRST if you don't the settings get confused and you will have to pay someone with VAG-COM to reprogram the height settings.
