Yihua 853AA with preheater but no hot air

[kripton2035]

  I bought 4 years ago a yihua 853aa station. it worked fine until now.
today, the preheater works ok, but the hot air seems like the gun is in the holder, that is it sleeps all the time.
I have 3 dashes "---" on the display, and the air flow is not flowing at all.


I opened the station, and the heat gun,
I checked the reed switch: it seems ok as it closes when I approach a magnet
I checked the thermocouple : it gives millivolts when I heat it
the heater seems not open : I measure 75 omhs between the wires.
I traced the wire from the reed switch on the board to the samsung micro controller pin : nothing is cut
I measured 5v on the micro controller pin, and zero volts when the reed switch is closed by a magnet


so I came to the conclusion that the microcontroller thinks the reed is always closed


do you have any other thing I could do/measure to be sure of that failure ?
or any hint that could make this board work again ?


thanks.

[kripton2035]

forgot to mention :
I checked the triac BTA16 seems ok, optocouplers moc3041 seems ok too