The cut out time is a diagnostic indicater of the fault. If it cuts out after 3 seconds the high voltage inverter board is faulty, an expensive part. If you fit a new one and it does the same then the faulty one has also blown an internal fuse you need also to replace, it will be on the pcb near the front of the oven , nowhere near the inverter board. Inverter board runs at 5000 volts and should only be attended to by a qualified microwave technician Cheers
Kenjo, December 2012
