whirlpool fault

hi,
whirlpool microwave
the numbers are very faint but they look like:
KM457
853825715291

the customer says that there is an intermittent fault on the machine.
it sometimes comes up with the word ‘plate’ on the display and will not work for a while.
any one got any ideas?
the turntable motor seems fine as does the plastic drive.
the customer said she always makes sure the plate is positioned correctly.
cheers.
john