Is there only one check valve on this machine to prevent siphoning from customers drain back into the sump? I was call here to service the dishwasher for a no pump out situation and cleared white cholesterol type build up from the drain hose. At the pump outlet port, I removed the check valve to view it for an obstruction and found none. Dishwasher pumps out ok, but water comes into same from customers sink. Drain hose is not on Y tail but rater a T with no ck valve in the drain. I was told there may be a ck valve missing inside the dishwasher drain hose? Please advise. PS the drain hose is looped at top of cabinet from drain then back down to pump. Thanks. 
The check valve is screwed onto the pump ([COLOR=#0066cc]AP2910020[/COLOR]) then the drain hose connects to it. So if you looked at it and didn’t find one, then it probably broke loose and flushed out with some recent drain water. Many of the newer drain hoses use a duckbill check valve mounted inside one of the ends. Given the buildup you found in the hose, this unit may be in need of a new check valve and drain hose.