I am installing anew washer kenmore HE4T drain pipe I was told if the trap is to low like under the floor that the water falling that far will empty the trap . Is this true ? And do I need a vent also.

Trap for washer standpipe must be located above floor, and you will need a vent for the trap.

My area requires a standpipe between 18 and 30 inches. Two inch stand pipe, p-trap and drain. Vent for two inch trap arm must be within 5 foot of trap. Hope this helps.

In my area, trap has to be located above the floor, bottom of the trap no less than 6" above the floor and stand pipe 30" tall.