bosch D/W sms40c02gb error09

Hi
there has been a few posts about this fault. i replaced the heat pump,the heater was open circuit. Tested d/w on quick wash all fine.
Customer used it first time reported back to me e09 fault. went back yesterday and the element on heater pump open circuit.
Problem i have is why is this happening and i have 2 other customers with same fault, element open circuit. i dont want to order pumps and fit if this keeps happening.
Any ideas as to why this is happening?