Several options are available to you. You can install Openmanage Server Administrator and check from this what is untoward.

You can run DSET utility if you have it or you can run log2txt from floppy.

Have you checked the fans for corrrect rotation. Common problem I've seen is the fan in front

of CPU0 running slow causing this problem.

I have a similar problem with a used PowerEdge 2550 I just bought, in my case running Windows Server 2003.  The status light on the front blinks yellow slowly.  I installed OpenManage Server Administrator 4.5 on it and it reports everything is healthy and nominal except "Hard drive fan 1" reading 0 RPM.  System fan 1 through system fan 3 all report reasonable RPMs.

Looking at the online manuals for the PE2550, it appears there are only three system fans, though the connector they plug into is set up for four:  http://support.dell.com/support/edocs/systems/pe2550/en/it/05mkhc70.htm#1033873

I can find no hint that there should be anything connected to that fourth plug coming off the fan wiring harness plugged into the interposer board.  Curiously, if you Google "poweredge 2550" "hard drive fan 1" you come across the release notes for an update to software called OpenManage Server Agent 4.3.0B which mention correcting the name of "hard drive fan 1" to "system fan 4":  http://ftp.dell.com/sysman/UpdateA00.txt  I find this curious because that update is from 2002, yet OpenManage Server Administrator 4.5 from 2005 doesn't have that fix, it still refers to "hard drive fan 1".

I would love to either find out where the fourth fan is supposed to be located so I can install it, or how to convince the software that it shouldn't be installed so I stop getting a false fault indication via that flashing yellow light, and via the red X next to Fans in OM Server Administrator.

Here's the software installed on the system, from the ISM 4.5 CD and the PowerEdge Updates 4.5 CD, as reported by OM Server Administrator:

Dell OpenManage Server Administrator   Version  4.5.0 
 Contains
   Broadcom SNMP Agent 1.0.0 
   Common Storage Module 1.2.0 
   Data Engine 5.3.0 
   Hardware Application Programming Interface 5.3.0 
   Instrumentation Service 5.3.0 
   Intel SNMP Agent 1.56.1 
   Secure Port Server 1.0.0 
   Sun Java Runtime Environment 1.5.0 
   Diagnostic Service 3.4.0 
   OLD Common Components 1.2.0 
   Server Administrator Framework 2.2.0 
   Storage Management Service Integration Layer 2.2.0 
   Instrumentation Service Integration Layer 2.2.0 
   OpenManage Inventory Collector 2.3.0 
   Storage Management 1.2.0 
   OpenManage Tools 2.2.0

-- Dave Hart

hi,

is fan 4 not your litle fan thats build in the protection case of your memory? there is one in my PE2550.