Siemens S40 "Fault 1

Hello,

I have here a S40 which shows "fault 1" shortly after the "heats up" message in the display.
The "usual" suspects like temperature sensor and overtemperature fuses
I checked and are OK.
If I manually set the plug connection of the heating blocks behind the triac to "N", the block heats up and the temperature sensor resistance
becomes smaller.

Nevertheless

error 1

comes again

after a few seconds. I
could not detect an input signal at the optocoupler in front of the heating triac, because the
heating
should
be started.

Are there other possible sources of error besides the electronics (and of course broken cables)? (I know from the Jura after the main switch on a complete initialization cycle, which seems to me here at

Siemens a little short

) Since the machine is just disassembled, must be installed in addition to the shell something else / magnetic switch closed so that it

can work?
Thanks a lot for your help
and ideas.
GreetingsRudi