[mwebb]

Originally Posted by Daox

I was thinking this over this morning and I'm really thinking its the O2 sensor. The code doesn't pop up for ~20 seconds after the car has been running. Thats just about as much time as you need for it to go into closed loop mode.

I had planned on changing it out last night but had problems getting the old sensor out.

probably not -
if the 02 sensor was that bad - it would code the system
you do not have a code for the 02 sensor ....
and fuel trims would be out to lunch.
ie; sum of LTFT and STFT would be greater than +/- 25% from zero trim adjustment

test , do not guess .
it is always cheaper and better er

i recommend that you ;
DO NOT add a new problem by installing a FRED's 02 sensor
Frakin - Ridiculous - Engineering - Design .... FRED's
OEM only

a scan tool with GM Enhanced Data will have Mode 6 testing results
for the 02 sensor (s) , those test results will condemn or exonerate the 02 sensor as the culprit .
the system may give the rear 02 sensor some trim authority , an exhaust leak near the rear 02 sensor can skew fuel trim values ,
do you have an exhaust leak near the rear 02 sensor ?
would NOT cause p0300 though ....