Neff HTHB86K Microwave oven.

Question

Neff combination microwave not operating in microwave mode ,LCD display reads E9. Both oven and grill facilities working correctly. Occasionally microwave will operate for set time period, no heat in food. Next attempt unit will stop halfway through time and error E9 and display : MW fault acknowledge with stop.
Would be great if someone could identify fault as I am fed up eating salads.