Besides the basic rotten egg smell they tend to give off when not working correctly, or the typical clogged cat which gives too much back pressure, my understanding is that there should be ~75*F difference between the input & exit of the cat if working correctly.
Seems I've read the ~75*F amount somewhere at a fairly reliable source.
BTW: Shame on you for not starting it more frequently.
I know....that's all you need is to hear some old fart chiming in scolding you...Anyway, hope it's nothing too bad to fix. Probably the cat though.