The 11,2 Dual and Quad Core machines definitely have the self-diagnosis LEDs.
But I'm not sure if the Dual 2.7GHz had this feature. I could be wrong, but my understanding is the 2.7 was based on the original G5 logic board design. The only LED you'll see light up is for when the perspex deflector door is open (or if the air deflector sensor board has failed).
[doublepost=1506863624][/doublepost]Just reading through the
powermac_g5.pdf Service Manual...
Step through the section titled:
Startup Failures for Power Mac G5 (June 2004 Dual 2.5 GHz/ Early 2005 Dual 2.7 GHz)
(Page 188)
One thing that hasn't been mentioned on the thread yet is to try
Resetting the PMU on the Logic Board (Page 176) This may do the trick, but the processor reseat will more than likely be required.
I don't have any experience with the LCS units, but from what I understand, most cases of failure on these early Liquid Cooled G5s was gasket/o'ring fatigue causing leaks of the highly corrosive coolant, which ultimately damages other sensitive components.
Are there any visual signs of coolant leakage or corrosion?