How do you know if you're compressor is on its way out?
 

Most threads I see usually have people with compressors that are done for already or are fine but something else is wrong. I've been trying to track this issue where my higher ride height selections will grey out on and off while I'm driving. It doesn't do it every drive, but fairly often I'll lose the ability to select lift when in comfort or to select lift, comfort, or automatic when in dynamic. It'll come back on shortly after but then disappear again, ad infinium, until the trip is done. I checked my compressor output, and it at rest was 8 bar, while lifting the car 9 bar, and when I lowered it, 3 bar. I already rebuilt it after it lost the ability to even lift the car after the third strut failure(FR for the second time.) I also get periodic compressor overtemp codes, but the car will hold its height for several days if unused. I know something is wrong, but I have no idea what, and the compressor pressure seems a lot lower than the 15-16 bar Gio said he sees on his. Does anyone have any ideas?

My post from October 2017: I am having this same G291 issue. There is a way to see the pressures associated with each axel and the accumulator. Using VCDS go into 34- Level Control --> Security Access -->08367 --> Basic Setting --> Channel 20, 21, and 22 will drain your suspension. However, if you quickly press and repress on/off/next it will flash the actual current pressure in bar, so you know an initial starting pressure and ending pressure to see if there is a leak over time. My only caveat to this is that in any other mode besides jack mode the car will self adjust at 2, 5, and 10 hours if necessary (pressure drop due to cooling), the car senses this and adjusts as necessary. So if your looking for a leak or want to verify the system isn't leaking leaving it in jack mode, read the values, and then reread the values after 12-24 hours. I used this method to determine that it is either the valve block G219 sensor going bad or a weak compressor. I am looking into a Mercedes S-Class suspension compressor and the feasibility of using it in place of an Audi one. They are the same manufacturer and specs, but the Mercedes compressor is a fraction of the Audi equivalent. For those who have their doubts: http://forums.quattroworld.com/allroad/msgs/34738.phtml it is also noted in the D3 Suspension SSPs that the Allroad and A8 compressors are equivalent.

Update on using a Mercedes S-Class suspension compressor: https://www.audiworld.com/forums/a8-...essor-2960928/

Note: I have not swapped my compressor yet for the Mercedes S-Class equivalent, so this method is untested, as I have solved the intermittent graying out of suspension options and compressor over temp by simply putting the car in jack mode anytime I leave the car.

I appreciate it! I suppose if it is my compressor, it'll fail entirely eventually, and then I will know for sure.

Hmm, well as for selecting Lift, if you are talking about it being greyed out when driving... i know the car greys it out automatically above a certain speed... it will gray some out out when in jacking mode i think too.

If you keep thinking and suspecting the pump, just buy a new one to be sure and get rid of the problem once and for all. You did mess up the suspension the last time you fixed it some how.
Trace back to the last fix.
Cheers,
Louis

It wasn't a problem from a botched repair, the issue was present before I made any repairs to the air suspension, unless you count the double front strut replacement before I ever got the car. I'm trying to be certain that is the issue before replacing the pump, because I have a limited budget.

All valves are 1 way valves unless released by the solenoids, the pump doesn't have to pump so much to fill the tank if all lines are airtight.
If there's no warning lights, suspension doesn't go low, you can put in lift mode to jack up the car when standing still, then it should be fine. Why worry about the lift mode greyed out? It's an old car with all types of aftermarket parts installed, who knows.
Cheers,
Louis

It isn't an emergency, but I'd prefer to figure it out if I can just in case it is something that could cause more serious concern if the failure mode gets worse. The car does seem to hold height just fine, no sign of it losing it.

As stated above, lift will gray out above 50 or 55 mph. Alternatively, if you are in lift mode and reach that speed, it will drop out of lift. Does Lift, Comfort and Auto gray out if you are driving in Dynamic? If so, then your pump has an issue.

Yes, as mentioned in the first post, it does lose comfort and automatic if in dynamic.