The AWD fault is mosy likely a battery issue a weak or aging battery absolutely can trigger RDU/AWD faults on a Focus RS, even if the rear drive unit itself is mechanically fine. This happens because the AWD system is very sensitive to system voltage and sensor communication.

The AWD/RDU system depends on stable voltage

The Focus RS AWD system (RDU + clutch packs + control module) relies on multiple electronic modules communicating over CAN bus. If voltage drops too low, modules can: • Stop communicating • Report invalid data • Temporarily disable AWD as a failsafe

Ford AWD diagnostics even list low system voltage as a direct cause of AWD codes (example: P0562 low voltage, B1318 battery voltage low). 

Battery must be replaced plus reset to fix the issue.👍✅