if you have detergent running down inside of door after load is done .
It sounds as if you have Either no water coming into d/w during fill cycle OR wash motor is not circulating water around inside of tank
Do these two possibilities match up to the "Wash Arm" error code?
Yes ,, wash arm error is telling you it is not reconizing arm is turning and with out water it will not turn ,
The water pressure is what makes the spray arms turn, if you have no water or if the wash motor is not making water move,then no spray arm moving