ST165 Codes 21 and 52?

Interesting problem, when I start my car, it idles at about 2k for about 15 seconds, then the idle slowly drops, and gets really rough, until it finally settles around a bouncy 350-400 RPM. The CEL was on, so I checked codes and came up with 21 and 52. Now, I didn't think an o2 sensor problem (21) would cause problems at idle because I thought the ECU didn't look at the o2 sensor until around 3k RPM. The knock sensor (52) however seems to me the more likely cause since the engine does behave like timing is being retarded when it starts to die off. Any thoughts? TIA.

The ecu uses the O2 sensor until you reach wot [closed loop operation]. Once it reaches wot, it goes to the fuel maps [open loop]. So yes, the ecu uses the O2 sensor at idle [there are certain situations where it doesn't, like cold start].
Also, the ecu shouldn't be messing with timing too much at idle, bad knock sensor or not. Definitly not enough to cause that low an idle.

How old is the O2 sensor?
Does it do this only when cold?
I would also check out the isc [idle speed control vavle].

The O2 sensor is probably original to the car (145k).
It does this all the time.
I've tried unplugging the ISC with no change.

I would change the O2 sensor.
If you unplug the isc v, it won't work. That is [possibly] the problem now...

But yeah, change the O2 sensor first.