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Panasonic NN-8507 microwave, FFFF?



Jan 1, 1970
Fellow techs,

I have this Panasonic NN-8507 microwave combi in for repair. Every separate
function seems to work.

But when i use the combination button, micro and heater together, it seems
to go well for 4 minutes...then on the display i get 'FFFF' and the
microwave stops functioning.

I can only see 2 temp. controllers and 1 humidity(steam) sensor. The
temp.controllers seem to function allright using a lighter to heat them up
and they both switch off when getting too hot.

What else could cause this FFFF in the display?

(ps: I am an electr. engineer and thus aware of the dangers inside a
microwave, dont worry.)