DD60DCHX7

I have a F&P DD60DCHX7 which is showing an E4 error code which i thought was a draining problem,
When we test it , it all works fine and goes through the test cycle without showing the error, there is no blockages and the motor is clear and seems ok.
The customer had a new drain hose so we fitted that to keep them happy however it still does it at least once a week,

Any ideas ?

Simon