I have a General Electric Potscrubber dishwasher. When the cycle is finished, the water does not drain into the sink.I turned the unit upside down to look into the bottom. The motor turns freely. The instruction sheet I found in the bottom of the unit said to remove the filter screen and check the piston valve. I looked at what I thought was the piston valve, but I'm not sure how to check it. The line to the sink seems to be clear. The model of my unit is GSC3230Z03WW. The serial number is AZ627212A.

Whenever an appliance does operate its pumps/valves heaters correctly for part of a cycle then doesnt at another point, it is a problem with the controller of the sequence in the cycle.. On older devices this can be a rotating knob that slowly runs to end of cycle triggering events as it turns, by tripping blade switches near the rotating item. On these the switch trigger points can be inspected and sometimes repaired. On newer items the cycle is controlled electronically, and you have to replace the whole control unit, which is usually a large expensive part that includes the display and all buttons. Try getting a used control unit from Ebay and swapping it out, or consider replacing the whole appliance.

So due to this water is still left in the bottom correct? you would have to remove the drain line where it connects into the sink area and also check inside the hole that the hose connects into the sink for any blockages if there are none found here then you would deinstall the dishwasher pull the unit out and check where the drain line connects to the pump. If you also found nothing in the other end of the line then you would disconnect the pump and assure nothing is stuck in the pump and that the impellar can spin freely. lastly you would reassemble the pump and drain hose but you would drain the unit into a bucket and still have the unit pulled out then start the dishwasher and assure there are no leaks. IF you have any further questions message me back Thanks Rick

Yes water does stay in the sump all the time. This is normal.

Try running a cycle with a cup of Vinegar or Baking Soda in the dishwasher.

Also check to make sure that your drain line (under the sink) is higher than the sink drain and not allowing some sink drain seepage back into the dishwasher.

Sounds like too much water leftover.

Give me a make and model number of the dishwasher.

Check that the drain hose isn't kinked, and that where the drain connects at the garbage disposal at the sink isn't partially clogged. And, is the garbage disposal clean/clear of food stuff's.
I had a similar thing happen to me a few weeks ago, and it turned out the kitchen sink drain line was clogged.

Its likely to be one of the following

a) a partially blocked drainhose
b/ the solenoid on the drain valve not opening closing probably
c) a blocked drain valve with a piece of bone or debris

This article should help you:

Dishwasher Basics The main parts of a dishwasher are: Control mechanism
The control mechanism is located inside the door behind the control panel. Many units use a simple electro-mechanical system: a timer determines how long each part of the cycle lasts and activates the proper function at the proper time (such as the detergent dispenser, wash spray and draining functions). Units that are more expensive might have a computerized control system. Modern units also have a door latch that must be closed for the unit to run. Some also have child safety locks. Intake valve
This is where water from the home's water supply enters the dishwasher. The unit's pump doesn't pump the water into the basin – when the intake valve opens, water pressure drives the water into the unit. Pump
An electric motor powers the pump. During the pump cycle, the pump forces water up into the spray arms. During the drain cycle, the pump directs the water into the drain hose. The motor-pump assembly is mounted beneath the basin, in the center of the dishwasher. There are two main types of pump:

• Reversible
  These pumps switch between pumping water to the spray arms and pumping water to the drain by reversing the direction of the motor. Reversible pumps are usually vertically mounted.
  -->Reversible pump
  
  
• Direct-drive
  The motor runs in one direction, so the direction of flow is switched from spray arms to drain by a solenoid that opens and closes the appropriate valves or switches one hose connection to another. Non-reversible pumps are usually horizontally mounted.
  -->Direct-drive pump
  
  

Dishwashers can be installed in either a portable or a permanent configuration. Portable units have finished sides and top that can be used as a countertop. When not in use, the machine sits in place next to the wall. When it's time to run a cycle, the unit can be rolled on casters over to the sink, where it connects to the faucet and plugs into a nearby outlet. In a permanent installation, the dishwasher goes underneath the existing countertop and bolts into place. Hoses underneath the kitchen sink connect directly to the hot water line and the drain line, and the unit usually plugs in under the sink as well. Both types of installation require a 120-volt grounded line.

Check the following:

* Loose or mis-positioned hose clamp at the drain spur connection

* Rotted or brittle hose

* Split hose

* Cracked joint at the drain spur connection or split drain spur fitting

pull your water drain line out and place in a bucket on the floor. then try to drain it again. alot of time the hose get an air gap or blocked with gunk.

Try adding bicarb of soda/baking soda and vinegar in the detergent dispenser. Bicarb fizzes off hardened particles and vinegar strips off grease and aids rinsing.

Two things to consider....clean water or dirty water?
Clean water usually indicates the fill valve is not shutting off all the way and the fill valve is "weeping" water in all the time = new fill valve time!!
Dirty water usually is not a dishwasher problem but more of a plumbing issue. Often the sink drain that carries the sinks water outside is not fast enough and some of this sink water is backing up into the drain hose and filling up the dishwasher through the drain hose. The drain from the sink to outside needs attention as this type of a problem indicates a slow house drain for that sink.
The dishwasher drain line should be clamped to the top underneath part of the counter before entering the sink drain plumbing, this may help the water from coming back inside the dishwash
If the cycle doesn't complete, check these:

Water-heating cycle
Timer
Water-heating cycle Many dishwashers heat the wash and/or rinse water to a higher temperature, as the cycle calls for it. In these cycles, the dishwasher pauses after it has filled with water, waiting for the water to reach the pre-set temperature. What happens next, depends, as follows:

• On some units, the pause is timed and the unit continues after the allotted time.



• On others, the cycle can't continue until the water reaches the higher temperature. If you have this type of dishwasher and the water isn't being heated (say, because of a problem with the heating element or thermostat), after the unit stops at the heating cycle, it never continues. When you repair the heating-system problem, the "cycle problem" is fixed too.

Timer Timers don't often fail. But if every other part of the dishwasher seems to be working properly and the timer knob seems to be stuck in one place--doing one function continuously--the timer may be at fault. Dishwasher timers can't be repaired. If yours is defective, replace it. 

make sure the drain hose loops up then into the drain connection it's like a upside down "p" trap on a sink that way the water doesn't flow back to the dishwasher