Will O2 always throw CEL if bad ?

[Alero 2 Envy]

I have a 2.4 and got a rough idle but my car is not throwing any codes. I am thinking motor mounts as it only idles rough (Vibrates & Shakes) in Drive, 1st, 2nd and 3rd, does not in idle or reverse. It also only has the rough idle once warmed up to normal operating temp. Lately I have noticed I am getting valve chatter or detonation under light loads (low RPM loads) for example going up a hill in low rpm will cause the pinging im hearing and sometime it bucks or studders. Other than these issues under light load the car runs & drives great.

I wonder if a bad CAT or 02 will cause this even though I have no CEL ?

[jackal2000]

the o2 sensor has a tolerance. it can be on the extreme limit of going bad without throwing the MIL.

[Alero 2 Envy]

Valve chatter fixed !

Bad knock sensor connection. It was unplugged somehow, plugged back in and runs good. Still got the rough idle..