In the past month, I've been hearing a pretty loud crack/pop after I turn off the engine. I narrowed it down to the Front Cat making the noise every 1ish mins.
Is this a sign the cat is on it's way out?

-Mpg:21 (can't break 21mpg even w/90% Hwy on 91)
-Both Upstream 02 replaced recently
-Power: seems about the same.

 

There is chance its breaking apart. They either break apart or clog and if it was clogged, you would notice a power issue.

If you narrowed it down to the precat, I would either replace it or take it out and gut it (and install a non-fouler on the downstream o2 sensor.

 

Not sure I can gut it since I have a Smog Check coming up in 7 months. Wonder if I need to replace all the cats or just the precat up front? I know 02 sensors are replaced in pairs .

 

if one is shot, the idea is the other is close behind or the same, same logic with the O2 sensors, you can gut and use a O2 extender or get a O2 sim (give or take $100 and sometimes hard to find, sometimes not)
or keep it simple and expensive and just replace.

if you hear this noise consistently maybe have a shop check it out instead of assuming precats too, ya never know

 

engine pinging, underhood heat, and rotten egg smell are usually associated with clogged cats.