Sounds like bad motherboard to me. Have you downloaded the Z600 service manual (HP Z600 Workstation Maintenance and Service Guide)? It may give you additional information on interpreting the led's.
Do you have a lot of experience with Z600. I posted a thread and I've gotten no response. If you have any information or know a sys admin who might have an answer please reply to post "
Z600 workstation won't auto boot after power loss "
Thanks