The BK manual explains that the cat probe is an indicator, generally it reports temps that occurred 10min past (I think) so if you have a ripping fire and the probe is just below active zone, I shouldn't hurt engaging it. Now if your smoldering don't engage, add more air.
We're told that the cat lights off when flue gas temps are 500 or higher. Sometimes even lower with the steelcats. I have a flue temp probe meter and that bugger is often well over 500 but the cat meter says inactive. I've been tempted to flop the bypass despite what the cat meter says but I want to treat the cat really well so I wait. I've even begun turning the stat down to slow the fire while the cat probe rises.