Not turning on, only diagnose LED#1 is on

Hi guys,

if have a 2009 iMac A1311 EMC 2308. Its not powering up.

• diagnose led 1 is on
• 12V on pin 4 is present
• after pressing power button, 12V is present on pin 11 but diagnose led 2 stays off!
• sometimes, the 12V on pin 11 (run mode power) breaks down after a few seconds, but not always!
• power on request signal (pin 12) has 0V after pressing the button, arround 4V before pressing it (should be 12V while pressing)

In the manual it says its probably the PSU. But I thought the power on request signal comes from the logic board?!

Does anybody has an idea?

Thx in advance!

Greetings from germany, Ruben