O2 sensor

[Alfisto@domain.elided]

Before I lay out molte dollares on a new oxygen sensor for my 24V LS, what 
are the operational clues to to look for that one might be going bad? 
Obviously it will have an efffect on smog values, but will it send wrong 
mixture signals to the ecu when the motor is cold, as apart from when warm ? 
Occasionally when cold my car start fine, but then will act as if it needs 
more choke to bring the revs up to be able to move off smoothly. Having an 
auto tranny, it is not possible to add more revs so it hunts and acts up 
until it reaches about 1500 rpm and then runs fine. When it has warmed up the 
engine runs like it should from start. I suspect the O2 sensor might be at 
fault, but could the cat. converter innards create any problems when cold, 
that would disappear when warm ? 
Smog values less than a year ago were well within legal limits.
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