whirlpool 6th sense dishwasher F4 code what does this mean?

you can set the dishwasher on the program you want to use it works you can hear the pump working pumping water in but when it comes to emptying the pump is working but the water isnt drain ing and a f4 code comes up on the display i have checked the mess filter thats clean i have checked the u bend to see if this is blocked they are all clean is there somethink simple i am missing the machine was 18 months old when this problem accurred please can someone help