Ge Triton Xl Won't Drain

I have the GE Triton XL # GSD6660G00SS. The water is not draining and sits in the bottom of the DW. We have checked for clogs at the hose and for clogs or particles under the bottom rinse arm and in filter basket. After reading several other post, I would like to check the piston nut assmbly, but can not locate it on my DW. Is there one on this model or could the problem be elsewhere?