how to fix fault?

I agree with CK. Unless it is a very up-market microwave, they are not worth fixing.
If the high voltage transformer has failed a spare is very expensive. Also the high voltage diodes are hard to get.
Possibly look out for a discarded similar model to rob for parts, but it may have the same fault.

Be very careful if you open your microwave up, as there are some large high voltage capacitors inside that can hold a LETHAL charge for some hours after switching the unit off!

Jo
December 2013

Usually turns out to be diode and capacitor or if that is OK then magnatron or interlock switches HOWEVER due to seriously high voltages I recommend NOT trying to repair it
yourself.unless it is a very expensive one you might as well buy a new one

CK
November 2013