The question is with the Flight Controls System "A" switch in the normal position, which is "A" ON
, the answer:
97-14-03 (Yaw Damper System Modification) which was due for completion in 2000
The Yaw Damper Coupler can turn the System "A" light ON
in the even that pressure to the Main Rudder PCU is less than 500 psi or when full pressure is commanded to the RPR and system pressure is less than 2,200 psi.
If you want to see what Mr. Boeing sez about this, Ref AMM 22-12-01