Chosen Solution
When I disconnect the internal screen the Imac starts only after cmd option p r after 2 chimes on a external screen When I connect the internal screen nothing happens. Cmd option p r only chimes once. The external remains black. The fans are rotating. I put in a new SMC battery. So I think the videocard is ok, as well as the power supply. 16 Gb RAM and a Intel 5 processor. Who can help?
Given the fact you are disconnecting and reconnecting the internal display and seeing a difference in the systems reaction its more likely the display has a problem. Check the cable connections for damage are you making sure the cable connector is not shorting out. You also have a set of on-board diagnostic LEDS what are they telling you?
Diagnostic LED’s Update (12/14/2021) @Jj Dallinga - So #4 doesn’t light up with the cables connected correct? If thats the case the displays T-CON board has a short. Underneath the backside metal shield you’ll find this board
Inspect it for burnt markings around the caps and coils.
