Where to begin? Trying to fix my brothers 2006 MY 4.2 Supercharged L322. These days I am a marine engineer but started out at a Volvo dealership in 1992. I have acccess to to an Autel and a Snap-On diagnostic from time to time from my freinds garage.
The car has had multiple issues over the last couple of years but now only has a C1A13-64 fault code with suspension fault coming up on the dash. It simply does not vent air correctly.
What has been done so far? Aside from replacing every single component, steering, suspension and braking wise (something he decieded to do to increase the sale value of the car)?? New airbags, all four corners as well as all new height sensors, new Ally air tank, full rebuilds of all three valve blocks with coil resistance checked and tested using a 9V battery. The Hitachi compressor was replaced recently with a new unit from Ebay.
I have changed out the new pilot exhaust valve with a used one, Stripped the compressor to make sure there were no manufacturing defects or assembly issues.
Using the Autel or Snap-On, when I manually deflate the air tank, it functions correctly but if I choose to deflate the whole system, the airbags do not deflate.
If I disconnect the short black hose from the resevoir valve block to the air drier, it responds better to lowering by the switch on the dash but the fault comes back once it has settled. The exhaust silencer has been removed temporarily and I have tried removing the springs from the plunger in the pressure relief valve to see if it will unseat easier.

It has a recent battery and I use a smart charger to keep the system +12.9 volts at all times. The steering angle sensor is functioning correctly. I have paid extra attention to wiring harnesses and multiplugs, tested every fuse, cleaned every earth point known. The only thing left is a replacement of the ride level ctrl module but i'm not convinced.

Questions and answers please!

Drying desiccant gone into the air pipes?

The original drier was a little degraded but the lines were blown out as the components were replaced before. Stripping things again now, no powder anywhere. Its like a mechanical issue that is confusing the system and causing a fault code, rather than a signal not reaching an actuator and throwing a code.