My SLSI fault would come on immediately after vehicle start and would not self clear. Once I cleared it with a Creator 310+ scanner it would pump a little then stop. The Indicator position light on the dash control remained illuminated during the fault indication.
It would appear that due to the raggedy piston ring and scored bore condition, the pump was unable to achieve proper pressure or overheated quickly.
Bagpipinandy says the pump in proper condition should make 265psi or about 18bar.
If one has an older air suspension system (all E53s are old at this point...LOL) you may as well do this repair because that old piston ring can’t run forever and the original ring material does not appear to lend itself to a smooth wear, the wear of the ring revealed pitting of the side which ultimately scored the piston bore
Mike