"Emergency" alarm on VIPRION 2400 blade - cause and how to clear?

On the face of it a bit of a basic question this, but I can't find an answer in the F5 documentation. We have a red flashing "ALM" alarm on a B2100 blade in a VIPRION 2400. Can anyone guide me as to 1) how to find the cause of the alarm (I've looked in /var/log/messages - nothing relevant there) 2) how to clear the alarm (we have no LCD module)

I understand it's possible the alarm is from a while ago and the cause is gone (the platform guide says "The Alarm LED might continue to display until alerts are cleared", but I still need to find the cause.

Thanks in advance,

Stuart