Z800 fans run @ power up

[CallofTheWild]

Changed the 2N3903 temperture sensor but no effect on the fans. They all run as soon as I apply power. No video output. Tried three different cards, nothing. Tried 2 different PSU's. Get 3 beeps using one PSU and 4 beeps using the other PSU. Both  PSU's work in other Z800 units. Changed the motherboard. Same problem, Changed the CPU's. Same problem.

Could the start button board IC cause this problem? 

Repair manual beep codes lists 3 beeps = CPU ; 4 beeps = PSU overload BUT no red LED flashes that I can see.

Does anything plug into the IO/IO serial plug to get PRE startup failure codes? (Like a car computer does)

I'm flying blind here...  H.E.L.P. please

[DGroves]

are the 2 power supplies the same wattage? a fully loaded system will require the 1250 watt unit instead of the stock 850

with that said, if the supplies work in another system and they past "BIST" self test

(attach just a pwr cord,  fan should spin and red light on = good supply)

i would suspect you are not connecting, or mis connecting a cable somehow or perhaps not inserting the video card corectly or a damaged pci-e slot (try another slot for the card, using a known good video card)

when you replaced the aimbent temp sensor in the power cable, did you  connect the two leads together on the transistor, and did you make sure the two leads were not swapped around?