Computer remembering errors of removed motherboard with new one installed

Experts, thanks for assisting so far. Here is where I am at this point. I will provide a lot of info now and will provide more as time permits. This is a Dell T7500 Workstation. 67FW8P is the Dell number for the motherboard. The original problem I had was that the larger of the two front facing fans would ramp up to maximum speed after 1-3 minutes. The environment is 68 degrees. The  cpu heatsink was/is not even warm. I have tried another identical cpu. I am using the very best heatsink compound. The first replacement board made no change whatsoever. Fan still acting up. I bought an unused mb and it did appear that it in fact is unused. It arrived with the cpu protector plastic piece. NOW, at this point, for the first time, the fans are not speeding up and the computer seems to be performing ok. However, on POST I am still getting the cpu fan error and now (!) it is saying that the hard drive fan is also defective. I replace both fans with other duplicates but it still shows these errors.  There is a connector on the MB that says HDD_TEMP. It has nothing connected to it. I checked photos of the MB before I changed it out the first time and there was nothing plugged into it before I received it.

This computer was/is capable of much more than is being utilized at this point.There is only one HD and two optical drives. It can accomodate two video cards, but at this time is only using one. It is an Nvidia Quadro4000.
It is an unused card that I just installed as a test. I will leave it in the machine.

I have been working on computers for many years and this, by far, is the most confusing and time consuming, brain busting one I have ever encountered. I just cannot figure it out. At least the computer seems to be operating if I totally disregard the post errors and F1 to boot up normally after that.

nobus suggests miswiring. Anything is possible. Perhaps an expert will see some indication of this in the photos that I intend to post "soon". I really DO appreciate both Experts assistance on this most unusual situation. I would be lost without you. This computer would probably be recycled if you were not helping me and the family I donated it to would be without a computer.

More later. I will check this thread often.

any diagnostic lights on?

Could you have cross-connected the cables for the system / disk fans? Looking at a photo of the mobo I can see two fan connectors next to each other in front of the CPU socket.

Also have you tried replacing Dell MY306 UF070 M199P Precision T7500 SAS/SATA Interposer Module & Heat Sensor? (the hard disk cable).

Experts....I have decided to give up on this. I am now getting post errors and intermittent intermittent indications that this should be taken to the recycle center. I thank you for trying to get it going for me, but I am burnt out after three motherboards, new P.S. countless hours of troubleshooting and grief. It is time to say goodbye to this item sorry to say.

Actually I think that is a good idea.

Sad to give up when you have spent so much time on it.

I really think you should try replacing M199P , the temperature sensor is in the backshell, photo of it at https://www.itcreations.com/view_product.asp?product_id=18783

andyalder...I do not show that part on any diagram of this T7500 nor can I find it in my computer. I don't recognize it at all. It DOES make sense that there is or should be a temp sensor since nothing is plugged into the hd sensor plug on the MB.