The archives and searching are lacking with the "how to fix" for what appears to be a pretty common failure if the central locking pump. There is a thread under the B5 S4 forum that shows the failure but doesn't describe the fix.
Symptoms of Failure: The door and trunk lock begins to slow and/or completely stops working. The central alarm still activated but the locks do not engage. If you check for fault code 01367 - Central Locking Pump Run Time Exceeded (Likely Leak)
Cause/Failure Mode: The central locking module's pump has a graphite compressor wheel that shatters into many pieces.
8L0 862 257 N (Valid for 1999-2001, please check ETKA or your car's equipped unit before ordering a replacement)
1. The central locking pump is located in the trunk on the passenger rear corner. In order to access it you need to remove the trunk floor
2. Remove the lower trunk plastic trim
3. Then remove the cubbie holder by pulling it straight up. The holder is attached to the liner by a plastic slide.
4. Pull the liner away. Note there are two snap retainers located right behind the passenger lights. (on the right side of picture above)
5. Disconnect the two spade connectors from the back of the liner
6. The central locking pump is located in the foam show here.
7. Pull back the foam to reveal the pump
8. Disconnect the hold back tie that is holding the cables and vacuum lines. This will permit easier access to disconnect the connections.
9. Disconnect the three electrical connections. To remove the vacuum connection use 2 flat head screwdrivers insert them as shown, and spread the plastic fingers in order to get the white plastic connectors off the pump air nipples.
10. Installation is the opposite of removal. Attach the connections, place back into position, reinsert foam, passenger liner, trunk trim, and cubby.
11. You will need to reprogram the keys to communicate with the new central locking controller. Reprogram them in accordance with the Tech Article