How do I detect if a pool is down in an iRule?

This issue is you want to determine if the server is down before the LTM does, that is a bit tricky. I'm wondering if you combined the LB failed logic with the pool reselect, instead of redirect, I think the redirect has to be executed based on a positive return value for whatever condition you are checking for, then again they both might and we'll be back in square one.