Dell T5600 when booted shows left memory fan error

Hi,

My T5600 when booted shows left memory fan error

Press F5 to execute the diagnostic program

The program showed one of the memory fan's rpm 3 numbers all were N/A

Another one shows the correct rpm number

But the left memory fan still running

I changed the pci fan to plugin the left memory fan slot 

The screen shows left memory fan and PCI fan failure

I ever tryed clear the event log and server rebooted immediately ,never show any error.

But I shut down the server, boot it again still shows left memory fan failure

It's like laughing me :(  ..........

Does anyone ever see this error and how to fix it.

Thx alot!!!