Water sitting in the bottom of a dishwasher after a cycle is a drain problem, and the cause is usually somewhere between the sump and the sink. Finding it means checking a short list of spots in order. This is the sequence we work through, from the simple to the mechanical.
Turn off the breaker that feeds the dishwasher, and the disposal's breaker if the two share a circuit. Close the supply valve under the sink. The sump holds water and the machine holds line voltage, so both have to be off before you pull the filter or reach into the tub. Confirm the power is dead with the panel open before you start.
Clearing a filter, an air gap, or a hose is a reasonable homeowner job. Replacing a drain pump, a float switch, or any wiring is not. If you see water spreading under the cabinet, or the machine has been sitting with standing water long enough to smell, stop and call a qualified technician rather than running another cycle. The parts inside the drain path sit against both water and wiring, and a wrong reassembly can leak into the base pan and damage the control.
A technician visit covers both the drain and the electrical side of the fault in one trip. Standing water that sits for a while can corrode the float and the pump housing, so the sooner the cause is found, the smaller the repair tends to be. One diagnostic covers the whole drain path rather than one part at a time.
Water leaves a dishwasher through the sump, past a filter and a check valve, into the drain hose, and then out to the disposal, air gap, or sink drain. Debris anywhere along that path slows the flow until water stops leaving altogether. Food particles, grease, and broken glass collect in the sump and in the low spots of the hose.
Pull the lower rack and the filter, and look into the sump with a flashlight. Remove any glass, bone, or seed that has settled there. With the power off and the supply closed, disconnect the drain hose and check it for a plug of grease or a sag that holds water. A hose with a low loop traps water, and debris settles at the bottom of the loop.
A new or replacement disposal ships with a plastic knockout plug in its dishwasher inlet. If the installer did not remove it, the dishwasher drain hose is capped and the machine cannot empty. The cycle runs normally and ends with the tub full, because the drain pump is pushing against a closed port. It is easy to miss, since the machine otherwise sounds completely normal through the whole cycle.
Turn off the disposal's breaker, then pull the dishwasher hose off the disposal inlet and look inside. The plug is a molded disc that you knock loose with a screwdriver, then retrieve from inside the disposal chamber. Run a short drain cycle afterward with the panel open to confirm the water leaves the tub. If you find shards of plastic in the disposer later, they came from a plug that was only partly removed.
A countertop air gap sits between the dishwasher hose and the disposal or sink drain, and its job is to keep sink water from siphoning back into the dishwasher. The cap and the passages inside collect grease, tea, rice, and food particles. Once they clog, water backs up into the air gap and then into the dishwasher instead of draining away.
Twist off the chrome cap, then use a small brush or a length of thin wire to clear both passages. Reassemble the cap and watch the gap during a drain cycle; water should pass through rather than stand in it. Check the short hose from the gap to the disposal inlet as well, since it kinks easily behind the cabinet.
The drain pump moves water from the sump out to the hose. Its impeller can be jammed by a shard of glass, a pit, or a piece of hard food, and then the motor hums without pumping. The motor itself can fail, and the control can also stop sending it power. You may hear the difference: the pump note drops when the impeller is working against something it cannot move.
If the filter is clean, the hose and air gap are clear, and the pump is silent at the drain step, the pump or its circuit is the likely cause. Some dishwashers use a single reversing motor for wash and drain. On those, a jammed impeller stops the wash as well, and a check valve in the sump routes water out on the reverse stroke.
Dishwashers use a float to sense the water level, and many also have a float switch in the base pan that trips when water reaches the bottom of the cabinet. A tub float stuck in the up position can make the control think the tub is full and skip the drain, while a base-pan switch that has tripped cuts power to the pump entirely.
Look for the float in the corner of the tub and confirm that it moves freely, with no debris holding it up. If water has reached the base pan, dry it out and find the leak that put it there before the machine will run a full cycle again. A float switch or base-pan sensor that has stuck should be tested and replaced by a qualified technician.
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