Chosen Solution
Recently I bought for 300 PLN a mac pro 8 core from 2008. I borrowed from my friend GPU - Radeon HD 6850 from Sapphire Computer powers on 3 of 4 diag led are on but the last one that should be on (GPU) is off. I boot from El Capitan HDD. GPU spins. I used even ASUS GTX 285 and there was still the same problem I also tried Quadro NVS 295 but it was as the only GPU that was causing RAM error. I noticed that after some time goes asleep. Thank you in advance for help!
Tanizna 300PLN :) got few of them as well. But - on Apple Radeon 2600XT works ? If no - then dead PCIe - if yes - possible dust in PCIe - use hoover + compressed air. Is 6850 Apple flashed? 6850 MUST 100% work in 2008 as I flashed them many times.
