Does the microwave work properly on any of the possible settings?
If not, I'm afraid the microwave probably has a major fault with the magnetron or inverter
There have been a couple of previous questions about this
There are some details within these about the parts and how to contact NEFF's Repair service.
Of course such electrical repairs should only b e undertaken by a fully qualified technician
This was the other link I was referring to:
HI Richard have tested the response in all 5 settings @1minute 90&180 countdown to 0 as normal. 360, 600,&1000 all countdown to 37secs & cuts out .The same test in all 5 settings @2 minutes 90&180 countdown to 0 as normal.360 & 600 countdown to 1min 37 secs & cuts out.1000 cuts out @1min 57 secs but is erratic.1000 does not return to zero on cutout. hope I have given usefull information.if you require further tests please advise. Regards Edward stead