How can I fix my Simpson Aquapulse 513 w/machine (pump fault)?

I have the 5kg model. Each cycle the machine faults at the end of the last wash and entering the spray rinse phase. The fault is 'PU' which is the pump. I can hear a humming noise but the water doesn't drain, it sits like this for a few minutes and then the fault 'PU' comes up. If I turn the machine off and on again, and skip through the stages to spray rinse, it will continue with the cycle.