Fanuc Spindle Topcard Issue Alarm 09

We try to get our turning machine back running with the spindle top card (a20b-1003-0010) power up with alarm 09 on the top card display.

i know this means heat sink overheat, i check fans and other causes mentioned in the manuals and didn't found yet.

what i tried so far:

-swapped fans from other machine.
-placed working top card of other machine in it, then it worked
-placed top card in the working machine and also gave alarm 09


so this should mean that the top card is the issue itself , not the fans or heat sink sensor.

my question is now if i can repair this and what is the component on the top card that can cause this alarm?
i don't want to replace the card for only this problem and trying to repair it.

the machine hasn't being used long so maybe oxidation?