how can i fix?

is it connected under a sink?. Water from the sink can feed in to the dishwasher if the drain hose is routed incorrectly.Dry the sump out in the dishwasher.put the plug in the sink and fill it with water.pull the plug and see if water starts to appear in the dishwasher sump.If a washing machine is attached to the same point under the sink it can also be pumped out in to the dishwasher if there is a partial blockage.If its stand alone it could be a faulty inlet valve

ck
March 2012