Is clustering enabled (cf status)?  That will turn on the fault led if it's not enabled.  What does "storage show fault" say?  Other than that, the fault light could be on due to a previous fault in the system that has been cleared.  Are you aware of any previous faults?

If you log in to the Service Processor you can run "priv set diag", followed by "system fru led show".  That command will list out the available fru's that you can check the status on.  For example:

SP nodeA*> system fru led show
Invalid command usage
Usage: system fru led show <FRU-LED-ID>

Display status of the specified <FRU-LED-ID>.

<FRU-LED-ID>:

        0 = SP Controller Fault LED
        1 = SP Heartbeat LED
        2 = SP System Fault LED
        3 = SP Fan 1 Fault LED
        4 = SP Fan 2 Fault LED
        5 = SP Fan 3 Fault LED
        6 = SP IOXM Fault LED

        all others are not valid

SP nodeA*> system fru led show 2
FRU LED ID 2 is off